John Tomac (born November 3, 1967, in Owosso, Michigan) is a retired professional cyclist who competed in multiple disciplines in road racing and mountain biking during a successful senior career that spanned twenty years. He is regarded as a mountain biking icon and was inducted into the Mountain Bike Hall of Fame in 1991. He is a true all-rounder, having won major national and international titles in four disciplines.

John Tomac first participated in cycle racing at the age of seven. He began to enter BMX events in and around Michigan in 1975 and continued competing in this discipline into his teenage years. By then, he had achieved title winning success at State and Regional level. In 1984, at the age of sixteen, Tomac won the National Cruiser Class title with the factory Mongoose team. He turned professional in 1985 and spent his last year of BMX competition contesting events as a privateer.

In 1986, Tomac made two significant decisions: he relocated to Southern California, and left BMX competition in favour of mountain bike racing. He continued to ride for Mongoose. By the autumn of that year, he had won his first two major mountain bike events: the Ross Fat Tire Stage Race in Massachusetts and the Supercross Mountain Bike Exhibition race held at the Los Angeles Memorial Coliseum. Mongoose marketed a Tomac Signature Edition model in 1987, which reflected his increasing profile within mountain biking culture. Tomac achieved a few more notable wins during that year, including a second win in the Fat Tire Stage Race and three National XC Series rounds.