In 1978, Stempel became General Manager of the Pontiac division and in 1980, he became Managing Director of Adam Opel AG, the German subsidiary of GM. In 1982, he became General Manager of Chevrolet. In January 1984, he became VP and Group Executive in Charge of the Buick-Oldsmobile-Cadillac group. In 1986, he was elected to the board of directors, where he served until he "retired" (the board of directors essentially forced him out) in 1991. Currently he is Chairman of Energy Conversion Devices.
He graduated from Worcester Polytechnic Institute in 1955 and earned an MBA from Michigan State University in 1970.