Johnson began his career as a systems engineer with the U.S. Air Force, where he worked on the development of advanced missile systems. This role provided him with valuable experience in designing, testing, and analyzing complex engineering systems.

Johnson joined NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. At JPL, he worked on the Galileo mission to Jupiter, which aimed to study the planet and its moons. He was involved in the development of the power system for the Galileo spacecraft, ensuring it could operate effectively in the harsh environment of space. He also contributed to the design of the Cassini mission to Saturn.

During his tenure at NASA, Johnson also worked on the Stealth Bomber project, contributing to the development of advanced radar systems and stealth technology. This work required a high degree of technical expertise and creativity, further solidifying Johnson's reputation as an innovative engineer.