He owned Hagan Racing and was known for winning the 1984 NASCAR Winston Cup Series championship with Terry Labonte.

In 1981 Hagan entered a Chevrolet Camaro stock car under the Stratagraph team name in the 24 Hours of Le Mans.

Powered by a modified 350-cubic-inch Chevrolet small block, the car qualified well enough in the IMSA GT class, but was sidelined by a crash caused by brake failure.

[2] Drivers were Billy Hagan, Cale Yarborough, and Bill Cooper; also listed by some sources was Bobby Mitchell.

The following year, he came 17th at 1982 24 Hours of Le Mans, always on Camaro V8 350 inch Chevrolet small block (#81 team Stratagraph inc.), paired with Gene Felton.