MacNabb, whose father supervised steel-mill construction, moved quite a bit in his childhood, including ten years in India. He finished up his education in America at the New York Military Academy and Illinois Institute of Technology. He was hired by Carnegie Illinois Steel in 1930 as a blast furnace engineer. He served in the Navy from 1943 to 1948 as an ordnance officer, including early work in barrage antiaircraft rockets tested to defend the carrier Midway from Kamikaze attacks. He worked for Sandia in New Mexico, and then Pullman, directing pioneering research in production and storage of cryogenic liquid hydrogen for the nuclear weapons program. He was hired by Convair in 1953, becoming Operations Manager for the Atlas development flight test program at Cape Canaveral throughout the development and flight career of the Atlas missile. From 1966 to 1975 he worked for General Electric as Manager of Test and Operations for its military space projects. He retired then, but continued to work as a consultant. His gregarious nature made him many friends throughout industry and government, who he could call on when necessary to get things done. He was a great motivator of staff, remembered in his obituary as "a colorful and energetic character who was in love with rockets and the thought of space travel".
Birth Place: Gary, Indiana.