Lyle had a very late start in professional boxing. He turned professional under Bill Daniels, with trainer Bobby Lewis. His first fight was at the age of 30 in Denver, Colorado, against A. J. Staples, which he won by knockout in the second round. Lyle went on to post a 19–0 record with 17 knockouts, and became the 5th rated heavyweight contender. He scored impressive knockouts over notables Vicente Rondon, a light heavyweight champion; hulking Buster Mathis; and won by unanimous decision over former WBA Heavyweight Champion, Jimmy Ellis. After dispatching of Buster Mathis, Lyle stated that he is looking for a match-up versus current undisputed champion Joe Frazier, saying "When I'm ready. I'm not taking any shortcuts." Yancey Durham, Frazier's manager and an interested observer, did compliment Lyle on his punching power. He was ranked #4 heavyweight in the world by WBA and the Ring Magazine. Lyle's undefeated streak ended on a one-sided decision to veteran Jerry Quarry: the latter gave one of his career best performances using a boxer/puncher style to create openings first, gaining the initiative using his greater experience. Lyle then lost to Jimmy Young in 1975. In a later rematch, Young again edged Lyle and went on to outpoint George Foreman in 1977.
