By 1947, United’s fleet continued to evolve, first with the Douglas DC-4, and then with the DC-6, which introduced pressurized cabin service for passengers. The DC-6 could also fly coast to coast, only stopping in Lincoln, Nebraska.

In the next decade, United’s fleet included DC-6s, DC-7s and Convair CV-240 aircraft.

In 1954, United became the first airline with flight simulators that had visual, sound and motion cues for training pilots. During the 1950's, after a series of fatal accidents , it helped pave the way for modern Air Traffic Control. Also in 1959, United received its first Douglas DC-8, its first jet aircraft. United ended up buying the Douglas DC-8 instead of the 707 and became the first airline to fly Boeing's four-engine, shorter-haul 720 jet.

United merged with Capital Airlines on June 1, 1961, displacing rival American Airlines as the world's second largest airline behind Aeroflot of the Soviet Union. The merger with Capital resulted in United operating the British-manufactured Vickers Viscount which was the only mainline turboprop aircraft ever flown by the airline. Also in 1961, United began operating French-manufactured Sud Aviation Caravelle jetliners and was the only U.S. airline ever to operate the Caravelle in scheduled passenger service. In 1968 the company reorganized, creating UAL Corporation with United Airlines as a wholly owned subsidiary.

Boeing rolled out the three-engine 727-100 jet, and United began flying the aircraft in 1964. In April 1965, United placed an order for 66 aircraft and options on another 39 with Boeing, making it the largest commercial order ever made by an airline at the time. In 1968 United Airlines was the launch customer for the Boeing 737-200, which made it the aircraft of choice for short haul services from then on.

United took delivery of their first Boeing 747s in August 1970, initially operating them on longer routes within the United States. A year later, United, along with American, were the launch customers for the McDonnell Douglas DC-10, which for many years served as a workhorse in both airlines' fleets. United sought overseas routes in the 1960s, but the Transpacific Route Case (1969) denied them this expansion; it did not gain an overseas route until 1983, when they began flights to Tokyo from Portland and Seattle. After years of focused work to bring about deregulation, the 1978 Airline Deregulation Act became law.

In 1973, the airline commissioned designer Saul Bass to develop a new logo and livery. At the time, there was no real logo for the airline and Bass noted that the brand direction was not clearly evident. The "tulip" logo of coloured stripes representing overlapping letter "U"s was in use at the beginning of 1974. The "Rainbow" (or "Saul Bass") livery, which was the first to feature the "tulip", had a primarily white fuselage, with red, orange and blue stripes along the "cheatline". This livery remained in use for 19 years, the Bass branding would remain until early 1993.

In May 1981, one week after rival American Airlines launched AAdvantage, the first modern frequent flyer program, United launched its Mileage Plus. In 1982, United was the launch carrier for the Boeing 767-200, receiving its first 767- 200s on August 19.

In 1984, United was the first airline to serve all 49 states with commercial airports when it started flights to Atlanta, Nashville, Memphis, Little Rock, Fargo, Casper, Jackson, and Charleston.

In 1985, United agreed to buy the ailing Pan American World Airways entire Pacific Division, Boeing 747SPs, and L-1011-500s, and flight crew staffs for $750 million. By late 1986, United flew to 13 Pacific destinations, most of which were purchased from Pan Am.

United Airlines decided to establish the United Express brand on May 1, 1985, to serve as the regional feeder network for United Airlines. In the wake of airline deregulation, United needed a strategic way to funnel passengers from smaller, regional markets into its major hubs to compete with expanding rivals. By partnering with independent regional carriers rather than operating the smaller routes itself. At its launch, United Express partnered with three initial regional airlines: Air Wisconsin, Horizon Air, and WestAir. These carriers utilized a variety of versatile turboprop and small jet aircraft suited for short-haul operations, most notably the British Aerospace BAe 146, the Fokker F27 Friendship, the Short 360, and the de Havilland Canada Dash 7 and Dash 8.

This regional strategy allowed United to maintain a dominant presence in these markets while focusing its own mainline fleet on longer, high-capacity transcontinental and newly acquired Pacific routes.

The airline grew it's fleet with the introduction of the Boeing 747-400 in 1989, to replace older models and open up new markets with the extended range of the "jumbo". For short haul flights, it also received the Boeing 737-300 and 737-500 in 1985 and 1990 respectively while the larger B757-200 arrived in 1990.

The decline of Pan American World Airways offered good opportunities for other airlines; in 1991 United purchased Pan Am's routes to London Heathrow Airport. In direct negotiations with the UK government United also obtained rights to fly to Heathrow from Chicago. However, the aftermath of the Gulf War and competition from low-cost carriers led to losses. In 1992 United purchased now-defunct Pan Am's Latin American and Caribbean routes and Miami gates, but United allowed months to elapse between Pan Am's demise and its launch of service.

At the request of then-CEO Stephen Wolf, in 1993 United completely revised its branding and livery, to a primarily grey and dark blue fuselage, with blue stripes on the tail. A custom Times New Roman font, reading "UNITED AIRLINES" in white, replaced Saul Bass's previous lettering style. The familiar "tulip" logo remained, although slightly smaller.

In 1994 United's pilots, machinists, bag handlers and non-contract employees agreed to acquire 55% of company stock in exchange for 15% to 25% salary concessions. United used the opportunity to create a low-cost subsidiary, Shuttle by United, in an attempt to compete with low-cost carriers. In 1995, United became both the launch customer of and first airline to operate the Boeing 777, and had significant input on its design.

During the 1990's United received its first European made aircraft from Airbus, the A320 in 1993 and the A319 in 1997 to extend its competitiveness and replace older B737. Also, in 1997, United co-founded the Star Alliance with Air Canada, Lufthansa, Scandinavian Airlines and Thai Airways. That same year, United opened its Southwest US hub at Los Angeles International Airport.

In 1998, Delta Air Lines and United introduced a marketing partnership that included a reciprocal redemption agreement between SkyMiles and Mileage Plus programs and shared lounges. This scheme allowed members of either frequent flier program to earn miles on both carriers and utilize both carriers' lounges. Delta and United attempted to form an even closer codeshare relationship, but this deal was effectively killed by ALPA (unions). The marketing partnership ended in divorce in 2003, but paved the way for a future alliance with US Airways. In May 2000, United announced plans to acquire competitor US Airways in a complex deal. The offer drew immediate scorn from consumer groups and employees of both airlines. However, the Department of Justice barred the merger on antitrust grounds in July. The two airlines subsequently formed an amicable partnership that led to US Airways' entrance into the Star Alliance.

As part of the September 11, 2001 attacks, two of the four planes hijacked were United planes. One aircraft was N612UA, a Boeing 767–200 operating as United Airlines Flight 175, flown into the South Tower of the World Trade Center. The other was N591UA, a Boeing 757–222 operating as United Airlines Flight 93, which crashed in rural Pennsylvania after the passengers fought back against the hijackers, and was suspected to have been directed towards the United States Capitol building. These atrocities lead the airline to change its business in many ways and begin a complex reorganization process. United was in a worse position than before because it had failed to keep costs under control, possibly due to giving its pilots pay raises of up to 28% in the summer of 2000.

Coupled with a battered network, the post-9/11 decline in air travel and skyrocketing oil prices, lead to the company posting a loss of $2.14 billion in 2001. In the same year United applied for a $1.5 billion loan guarantee from the federal Air Transportation Stabilization Board established in the wake of the September 11 attacks. When the IAM union failed to approve the loan guarantee the company was forced to seek debtor-in-possession financing from commercial sources to cover the expected future losses. Unable to secure additional capital, UAL Corporation filed for chapter 11 bankruptcy protection in December 2002. Blame for the bankruptcy had fallen on 9/11, which triggered financial crisis in all the major North American airlines, coupled with the economic slowdown that was underway.

The events of September 11, 2001, changed the world of aviation and it hit brutally specially on the U.S.A. airlines like United.

United continued operations during its bankruptcy, but was forced to cut its costs drastically. All city ticket offices in the US were closed down. The airline cancelled several existing and planned routes, and eliminated its entire Latin American gateway and flight crew base at Miami International Airport after March 1, 2004. Furthermore, they reduced their mainline fleet from 557 (before 9/11) to 460 aircraft. However, at the same time, the airline continued to invest in new projects, to remain competitive.

On November 12, 2003, it launched a new low-cost carrier, Ted, to compete with other low-cost airlines. The SARS epidemic in 2003 depressed traffic on United's extensive Pacific network. The spikes in the price of jet fuel ate away remaining profits United made. United took advantage of its Chapter 11 status to negotiate hard-to-cut costs with employees, suppliers, and contractors, including cancellation of feeder contracts with United Express carriers Atlantic Coast Airlines (which became Independence Air) and Air Wisconsin(which became a US Airways Express carrier).

In February 2004, the airline introduced the new Blue and White livery, commonly called the 'Rising Blue' livery, with the Blue Tulip on the tail to coincide with a new advertising campaign. Financial pressure on the airline was heavy. On December 9, 2004, the airline made history when UA869, operated by a Boeing 747–400, landed at Ho Chi Minh City, Vietnam. The scheduled flight from San Francisco via Hong Kong (SFO–HKG–SGN) was the first by a U.S. airline since the end of the Vietnam War, when Pan Am halted service shortly before the fall of Saigon in 1975.

Originally slated to exit bankruptcy protection after 2½ years in the third quarter of 2005, United requested yet another extension in light of record-high fuel prices. On August 26, 2005, the bankruptcy court extended the airline's exclusive right to file a reorganization plan to November 1, although it also stated firmly this extension would be the last. The bankruptcy court approved the restructuring plan on January 20, 2006, clearing the way for United to exit bankruptcy. On February 1, 2006, United emerged from Chapter 11 bankruptcy protection under which it had operated since December 9, 2002, the largest and longest airline bankruptcy case in the history of the industry.

United's management called for consolidation in the industry and looked for a suitor in 2006. The Wall Street Journal revealed in late 2006, that Continental Airlines was in merger discussions with United. In the years following United’s exit from bankruptcy, two large financial firms, Bank of America and Fidelity Investments, accumulated shares to become the second largest owner with an 11 percent stake in the company. The surge in jet fuel prices caused disruption to United’s impending start of non-stop long-haul services. Though the FAA had already awarded the SFO to Guangzhou, China to United, they postponed the launch citing high fuel prices. Other long-haul city pairs, such as its 2009 application to fly between Los Angeles and Shanghai (which began May 2011) were denied by the FAA.

In June 12, 2008, United announced it would charge $15 for the first checked bag, becoming the second United States airline to do so, the first being American Airlines. The charges, were created in an effort to offset high fuel prices. On June 28, 2008, United announced the cessation of several international routes including San Francisco-Nagoya and Chicago-Mexico City.

On June 4, 2008, United announced it would close its Ted unit, and reconfigure them for a return to mainline configuration, to compensate for the removal of United's Boeing 737s that were to be retired. That retirement plan included Boeing 737s and Boeing 747s, reducing the mainline fleet from 460 to 359 aircraft and furthering the airline's goal of cutting domestic capacity by 15 percent. On January 6, 2009, Ted ended operations converting its entire fleet into United mainline fleet. In January 2009, United announced a code-sharing agreement with Aer Lingus for flights between Washington Dulles and Madrid. Aer Lingus would operate the service, which is permitted under recent open skies agreements between the US and EU.

In 2009, United entered into an extensive partnership with Continental Airlines. This partnership included codeshare and frequent flyer agreements. As a part of the agreement, Continental left SkyTeam and joined the Star Alliance. On October 28, 2009, United flew its final Boeing 737 flight, as UA Flight 737. United's 737 retirement was particularly significant, as it was the U.S. launch customer of the 737 family in 1967, and operated variants of the type for 42 years. Despite this, the airline ended up regaining 737s with the merge with Continental in the near future.

On April 16, 2010, United resumed merger talks with Continental. The board of directors of both Continental and UAL Corporation's United Airlines reached an agreement to combine operations to create the world's largest airline on May 2, 2010. The combined carrier would take the United Airlines name but use Continental's logo and livery. On October 1, 2010, UAL Corporation (the parent company of United Airlines) completed its acquisition of Continental Airlines and changed its name to United Continental Holdings, Inc. Although the two airlines remain separate until the operational integration was completed (by mid-2012).

After the merger with Continental the process of restructuring began in order to combine seamlessly all the operations of the two airlines into one. After the fully integration and the award of one single air operator's certificate from the Federal Aviation Administration, the name of Continental disappeared from its planes and was relegated to the history books. The "new" United was born. The first few steps were to restructure its fleet and retire older models with new fuel efficient jets like the Dreamliners which started to arrive in 2012 with the Boeing 787-8 and soon after the 787-9.

The introduction of the Dreamliner in 2012, directly influenced United’s route expansion strategy. The airline announced a series of new long-haul routes enabled by the aircraft’s efficiency and range. These included services from Houston to Lagos, from Los Angeles to Tokyo Narita and Shanghai, and from Denver to Tokyo Narita, all beginning in early 2013. Temporary routes such as Houston to Amsterdam and London Heathrow were also introduced as part of the rollout.

The years 2012 and 2013 also marked continued integration between the legacy United and Continental operations. The airline worked to unify reservation systems, operational procedures, and branding. However, this period was not without setbacks; the integration led to disruptions, including IT outages and customer service issues, which affected United’s reputation. Despite these challenges, the airline continued to push forward with network expansion and fleet renewal.

By 2014, United's Dreamliner fleet gradually expanded, and its role in long-haul operations increased. The aircraft allowed United to experiment with new city pairs and optimize existing routes by reducing operating costs. At the same time, United continued to operate a diverse widebody fleet, including Boeing 777s and 767s, but the long-term strategy was increasingly centered on newer, more efficient aircraft like the 787.

During this period, United also refined its hub strategy. Major hubs such as Chicago O’Hare, Houston Intercontinental, Newark, San Francisco, and Denver were strengthened as key connecting points for both domestic and international traffic. San Francisco, in particular, became a major gateway for transpacific routes, while Houston played a critical role in connecting traffic to Latin America and Africa.

In terms of fleet strategy beyond the 787, United continued to invest in both narrowbody and widebody aircraft. The airline also maintained and upgraded its existing fleet, including Airbus A319 and A320 aircraft inherited from the merger, as well as Boeing 737 variants.

By 2015, United began to shift more attention toward improving the passenger experience, which had suffered during the merger years. The airline introduced upgrades to cabins, in-flight entertainment, and onboard services. While the most significant overhaul of its premium product would come slightly later with the Polaris business class announcement in 2016, which gave a high-quality onboard product.

Although the full rollout extended beyond 2016, the announcement signalled a major strategic shift toward enhancing the customer experience on international flights. The new product featured improved seating, better bedding, upgraded dining, and dedicated lounges, reflecting a broader industry trend toward premiumization and competition for high-value passengers.

By the end of 2016, United Airlines had largely completed its transition from a post-merger airline struggling with integration issues to a more stable and strategically focused global carrier. The combination of fleet modernization, especially the introduction of the Boeing 787 and the network expansion into new long-haul markets, together with a renewed emphasis on customer experience positioned the airline for future growth.