[2][3] After holding research and teaching positions at Harvard and Princeton Universities, he joined Hughes Aircraft in 1948 and later worked at the Guided Missile Division of the Ramo-Wooldridge Corporation.

In 1986, President Ronald Reagan selected Fletcher to administer NASA for a second time, to help the agency recover from the Space Shuttle Challenger accident.

Shuttle flights went into a two-year hiatus while Fletcher ensured that NASA reinvested heavily in the program's safety and reliability, made organizational changes to improve efficiency, and restructured its management system.

He oversaw a complete reworking of the components of the Shuttle to enhance its safety, including a redesign of the solid rocket boosters, and added an egress method for the astronauts.

He died from lung cancer on December 22, 1991, at Georgetown University Hospital in Washington, D.C., aged 72,[3] and was buried at Salt Lake City Cemetery.