He achieved the rank of captain in 1967, was designated a Naval Aviator in 1968 and was subsequently assigned to duty flying F-4 Phantom IIs with VMFA-333 at Marine Corps Air Station Beaufort, South Carolina.

He returned to the United States and an assignment as aviation weapons division officer with VMFA-251 at the Marine Corps Air Station at Beaufort, South Carolina.

In August 1979, he completed a one-year training and evaluation period, making him eligible for assignment as a mission specialist on future Space Shuttle flights.

During the mission, the STS-7 crew deployed satellites for Canada (ANIK C-2) and Indonesia (Palapa B1); operated the Canadian-built Remote Manipulator System (RMS) to perform the first deployment and retrieval exercise with the Shuttle Pallet Satellite (SPAS-01); conducted the first formation flying of the Orbiter with a free-flying satellite (SPAS-01); carried and operated the first U.S./German cooperative materials science payload (OSTA-2); and operated the Continuous Flow Electrophoresis System (CFES) and the Monodisperse Latex Reactor (MLR) experiments, in addition to activating seven "Getaway Specials."

Other duties were operation of the Geophysical Fluid Flow Cell (GFFC), Urinary Monitoring System (UMS) and the Ionization States of Solar and Galactic Cosmic Ray Heavy Nuclei (IONS) experiment.

In addition, crew members also worked on secondary payloads involving fluid research in general, chemistry and electrical storm studies.

Thagard served as payload commander on STS-42, aboard the orbiter Discovery, which lifted off from the Kennedy Space Center, Florida, on January 22, 1992.

Fifty five major experiments conducted in the International Microgravity Laboratory-1 module were provided by investigators from eleven countries, and represented a broad spectrum of scientific disciplines.

During 128 orbits of the Earth, the STS-42 crew accomplished the mission's primary objective of investigating the effects of microgravity on materials processing and life sciences.