In 1923 Charles F. Kettering and Carroll A. "Ted" Hochwalt recruited Thomas to work as a research chemist at General Motors (GM).
Thomas would spend the rest of his career with Monsanto, becoming a member of its board of directors in 1942, vice president in 1943, executive vice president in 1947, president in 1950, and ultimately chairman of the board from 1960 to 1965.