Theisinger graduated from the California Institute of Technology in 1967 with a bachelor's degree in physics. He joined NASA's Jet Propulsion Laboratory (JPL) as payload integration engineer that year and except for a three-year span in the early 1980s has worked at JPL since. Among the missions on which he has participated were the 1967 Mariner 5 flyby mission to Venus, the 1971 Mariner 9 orbiter mission to Mars, the Voyager mission to the outer planets of the solar system, the Galileo mission to Jupiter, and the Mars Global Surveyor orbiter.