Whilst serving with the Marines he won fourteen amateur boxing titles, and was deployed for two combat tours in the Vietnam War.
[citation needed] After a discharge from the Marines with the rank of sergeant, Foster turned professional, becoming the third Fresno boxer of note (Young Corbett III was a world welterweight champion and Wayne Thornton rose through the rankings to become a number one heavyweight contender in 1970).
[dead link][2] Standing at 6 ft 2" in height and known as "Big Mac The Knife from Fresno", Foster made his professional debut in 1966, winning his first 24 fights by knockout, and was named The Ring's Progress of the Year for 1969.
"[citation needed] Whilst being employed as a sparring partner for Sonny Liston, Foster was reputed to have knocked down the former world champion,[3] causing Liston two days later to work Foster over during another sparring session before handlers could intervene between the two men to stop it.
By 1970 Foster was ranked as the world's number one heavyweight contender and seemed set for a title shot, but his 24–0 winning streak came to an end when as favourite he was stopped in six rounds by the more experienced Jerry Quarry in June 1970.