He worked at NMAI for ten years, and served as Senior Counselor to the Director.

While working at the library, he served in many capacities, including the Chief of Stacks for three years and Assistant to the Director starting in 1919.

In addition, he served as an intelligence and policy advisor to the Director of National Intelligence and his Principal Deputy.

Caldwell served as a scientific aide to the Director of anthropology of the United States National Museum from 1943 to 1945.

Also during this time, he served one year as the Executive Assistant to the Director of the NRO.

He served as assistant to the Director, Bureau of Imports, War Trade Board in 1918.

From February 2011 to October 2011, she served as Counselor to the Director of OMB.

He next served as the technical assistant to the Director, Flight Crew Operations.

After leaving Congress, he served as special assistant to the Director of Foreign Operations Administration in the Berlin area in 1954 and 1955.

In 1952, while still at Harvard, he served as a consultant to the Director of the Psychological Strategy Board.