The First World War brought Fairfield out of its agrarian past by triggering an unprecedented economic boom in Bridgeport, which was the center of a large munitions industry at the time. The city population grew by over 50,000 people in 20 months, and the newly built wartime Remington Arms-UMC plant employed 20,000 people by 1916, producing 50% of allied ammunition cartridges.

The prosperity accompanied a temporary housing shortage in the city, and many of the workers looked to Fairfield to build their homes, and the newly founded Bridgeport Housing Company (run by the chamber of commerce), built more homes in Fairfield in the form of the Grassmere Village and Lordship Village developments.

The trolley and later the automobile made the countryside accessible to these newly rich members of the middle class, who brought with them new habits, new attitudes, and new modes of dress. The prosperity lasted throughout the twenties. By the time of the Wall Street Crash of 1929, the population had increased to 17,000 from the 6,000 it had been just before the war.

Even during the Depression, the town kept expanding. Fairfield became an attractive place for wealthy New York City residents to have a weekend home, vacation, or live in, looking for relaxation in a country atmosphere, with its beaches, fancy hotels, mansions. The grounding of a barge with two crewmen on Penfield Reef in Fairfield during a gale led to the 1st civilian helicopter hoist rescue in history, on November 29, 1945.

The helicopter flew from the nearby Sikorsky Aircraft plant in Bridgeport, Connecticut. Fairfield University was founded in 1943 by Jesuits.The opening of the Connecticut Turnpike in the 1950s brought another wave of development to Fairfield, and by the 1960s the town's residential, suburban character was firmly established.

Families from Bridgeport and elsewhere, looking to live to the suburbs, moved into Fairfield in large numbers starting after the Second World War. Fairfield grew from 21,135 residents in 1940 to 59,404 in 2000. Bridgeport's population fell in that same timespan. In 1971, Fairfield became the home of the corporate headquarters of General Electric (GE), one of the world's 5 largest companies at the time, which relocated from Manhattan.

On May 8, 2017, GE relocated to Boston, Massachusetts.