Warner was originally drafted 245th overall by the New York Rangers in the 1974 NHL Amateur Draft but he chose instead to sign a free agent contract with the New England Whalers of the rival World Hockey Association in 1978. He appeared in 41 WHA regular season games with the Whalers in 1978–79 and was retained by the club when the NHL and WHA merged in 1979. Before turning professional, Warner played for the Colorado College men's ice hockey team. He was also a member of the United States national team at the 1975, 1976 and 1978 World Championships.