He was a member of the Manhatten Project, the US atomic bomb development program. After an assignment at the Oak Ridge National Laboratory in Tennessee he moved his family to Los Alamos, New Mexico, where he eventually became an engineering design supervisor for one of the atomic weapons then under development. He then accepted assignments at the Atomic Energy Commisssion (AEC) and The Pentagon in Washington before moving into the private sector. In the mid-1950s, he joined the Nuclear Development Corporation of America in White Plains, New York, entering the emerging field of computer technology and development. Following stints in management at several large technology companies including Union Carbide, Teleregister, Western Union, General Precision Laboratories, and IBM where he was Manager of Advanced Applications Development, he became an independent consultant, working for both government clients including Oak Ridge National Laboratories and commercial clients including Mini-Computer Systems of Elkmsford, NY. In the mid-to-late 1950s, Leon volunteered at the Civil Defense Filter Center in White Plains, helping track and identify aircraft flying over the NY metro area. He devoted much of his free time to the study of UFOs. He convinced a Congressional Committee to force the Air Force to permit him to publish and distribute, in its entirety, the Air Force's Project Blue Book Special Report No. 14, the primary source book on the Air Force's findings related to UFO's.