Thomas Sim, III was born February 23, 1921 in Montreal (Quebec Province), Canada. In 1939, upon graduation from Westmont High School in Montreal, Sim enlisted in the Canadian Army, from which he received a medical discharge in 1941. From 1941-1945, Sim served in the Royal Air Force and was assigned to the Ferry Command with liaison duties between R.A.F. and the U.S. Army Air Corps.

After the war, Sim worked in sales at Canadair and Air Carrier Service Corporation. In 1959, he was named export sales manager for the Oklahoma City-based Aero Commander, a subsidiary of Rockwell-Standard (became Rockwell international in 1973). He was named director of marketing in 1960, and director of international sales in 1963.

Sim was a member of the Utility Airplane Council of the Aerospace Industries Association. The association was comprised of a majority of the aerospace manufacturers, and the council was a subgroup comprised of general aviation airframe and engine manufacturers. In 1963 and 1964, he was appointed to the Oklahoma Export Expansion Council, which focused on the advancement of the developing General Aviation industry.