Clervoy trained through elite French engineering and test-flight institutions before astronaut selection, including École Polytechnique, SUPAERO, and the French flight-test school at Istres. He joined the French astronaut program in the 1980s and later entered the ESA astronaut corps while training with NASA as a shuttle mission specialist — open definition.

His first flight, STS-66, was an atmospheric science mission that deployed and retrieved CRISTA-SPAS. On STS-84 he served as payload commander — open definition during a Shuttle-Mir docking flight, coordinating SPACEHAB experiment work and cargo transfer between Atlantis and Mir.

Clervoy's third flight, STS-103, was a Hubble Space Telescope servicing mission. He worked as flight engineer — open definition and robotics operator, using the shuttle arm to handle Hubble and support the spacewalkers. Across three flights he logged just over 28 days in space and became one of Europe's most experienced shuttle-era astronauts.