During the 1950s, Northeast operated the Convair 240, DC-3 and DC-6 throughout New England and in major northeast markets such as New York, Philadelphia, Baltimore, and Washington. It was during this period that Northeast inaugurated service to Florida using the ten DC-6 aircraft. Northeast ordered the ten turboprop powered Vickers Viscount in the late 1950s and operated it successfully for a few years until financial problems in the early 1960s forced the company to return them to the manufacturer. The jet age came to Northeast in 1959 when it leased a single Boeing 707 from TWA for the Florida route. In 1960 Northeast leased six Convair 880s that were operated between the northeast and Florida for several years. 

In 1965 the airline was bought by Storer Broadcasting, which made a commitment to rejuvenating Northeast with a new marketing campaign and new aircraft. Northeast ordered a fleet of 727s for its Florida routes as well as DC-9 jets and FH-227 turboprops for shorter routes. These new aircraft were known as Yellowbirds due to their two-tone yellow and white livery. In 1966, Northeast was the launch customer for the Boeing 727-200, which it began flying in August 1967. 

Despite a modern fleet and successful Yellowbird marketing campaign, Northeast remained at a disadvantage against larger competitors such as Eastern Airlines and National Airlines. By the early 1970s, Northeast's financial condition was such that it sought a merger or sale. On August 1, 1972, Northeast merged with Delta Air Lines. Northeast's contribution to Delta included access to the Boston market, which Delta did not serve under the then-regulated airline industry. In addition Delta assimilated the 727 into its fleet, an aircraft type it did not operate prior to absorbing Northeast. Delta went on to use this aircraft as the workhorse of its fleet during the 1970s and 1980s.