After college graduation, Brock signed a one-year contract with the Winnipeg Blue Bombers and was a starting quarterback in 1975. Nicknamed "The Birmingham Rifle", Brock is the only Blue Bombers player to win back-to-back CFL Most Outstanding Player awards in the 1980 and 1981 CFL seasons. In 1981, Brock broke Sam Etcheverry's 1956 record of 4,723 passing yards with 4,796 yards. Brock started his illustrious pro football career as a little used back-up quarterback for the Bombers in 1974. That season the team traded away their aging star passer Don Jonas to the Hamilton Tiger-Cats for the much younger Chuck Ealey. About midway through the 1975 season Brock became the starter and that resulted in Ealey being sent to the Toronto Argonauts.