Mankins was drafted in the twelfth round of the 1966 NFL Draft by the Green Bay Packers. He had also been drafted in the sixth round of the 1966 American Football League Draft by the Miami Dolphins. Mankins would go on to play for the Atlanta Falcons.
After playing in the NFL, Mankins would play 3 seasons in the Canadian Football League. With the Ottawa Rough Riders from 1968 to 1970, he would win two Grey Cup championships. He was traded to the Edmonton Eskimos in 1970, finishing his football career there.