William L. Mitchell was born in Cleveland, Ohio but grew up in Greenville, Pennsylvania. He got his love for cars from his father who was a Buick dealer and who frequently brought home Stutz and Mercer sport car trade-ins. Mitchell was passionate with cars and motorcycles. He loves to draw and drive them. He received formal training at the Carnegie Institute of Technology (now Carnegie Mellon University) in Pittsburgh in 1930 and at the Art Students League in New York City from 1930 to 1931.

He began working for General Motors on December of 1935. He was assigned at the Art and Color Section. After working for almost one year, he became the Chief Designer at Cadillac Studio in 1937. He succeeded Harley Earl's job as a Design Chief of GM. Bill Mitchell is best described as an artist and an automotive designer. His best work combined a sleek, sharp-edged appearance and the sensation of great power and all-out speed.

When GM was out of racing officially, he rented a garage together with his own team near GM's Warren, Michigan Tech Center. He exerted great effort in making Zora Duntov's Sebring SS mule car a SCCA C Production champion in 1960. at first no one believe him, his co-workers and people outside that he could make it. But it did not stop him because nothing is impossible for him, he strive hard to make it possible.

Mitchell worked for GM for 42 years, he became the Vice President of the Design Staff in 1977. He was responsible for designs from 1958 to 1977 that produced 72 million cars, including the Buick Riviera, Chevrolet Camaro, Pontiac Firebird, Cadillac Seville, and the Corvette, his personal favorite. He retires in 1977. After his retirement he operated a private design consulting firm, the William L. Mitchell Design, from 1977 to 1984.