He returned to the University of Southern California to teach freshman mathematics while obtaining his master's degree in business administration which he was awarded in 1939.
During World War II Gardner's work at the California Institute of Technology focused on rocket and atomic bomb projects for the Office of Scientific Research and Development.
In early 1955 most of the Eisenhower administration assumed that America had a strong lead over the Soviet Union in strategic technology and felt no particular urgency for the ICBM programs.
Eisenhower requested a briefing and, on 28 July 1955, Gardner, von Neumann, and Schriever made a presentation to the President and the National Security Council.
As a result, the National Security Council recommended the ICBM be designated a "research program of the highest priority" which the President approved on 13 September 1955.
All three services developed plans and the interservice rivalry led to a compromise with the Air Force building the Thor and the Army and Navy in charge of the Jupiter.
Gardner played a major role in establishing the U. S. Arms Control and Disarmament Agency and was named to its General Advisory Commission on 1 March 1962.
At the time of his death on 28 September 1963 in his home in Washington, D.C., Gardner was actively participating in Project Forecast, which was to chart the future course of the Air Force for the next decade.