Mantz also made 12 NASCARGrand National starts from 1950–1951 and 1955-1956. He won in his third NASCAR race, the first Southern 500 held at Darlington Raceway. Mantz' Plymouth was the race's slowest qualifier, but because he qualified on the 9th of 15 days of time trials, he started 43d in the 75 car field. Mantz fitted truck tires which did not wear quickly or blow out, while his competitors had to stop often to pit for new tires. It was his only NASCAR win.
As of 2010, the speedway presents the Johnny Mantz trophy to the winner of the Southern 500, now held each May on the evening before Mother's Day.
The Indianapolis 500 was part of the FIA World Championship from 1950 through 1960. Drivers competing at Indy during those years were credited with World Championship points and participation. In the 1953 Indianapolis 500, Johnny Mantz drove in relief of Walt Faulkner. As a result of this shared ride, Mantz participated in 1 World Championship race, but he scored no World Championship points.