During his 10-year NFL career, this 6', 202-lb running back rushed for 7,274 yards and 74 touchdowns on 1,727 carries for 4.2 yards per carry. He also caught 190 passes for 2,281 yards and 13 touchdowns. On special teams, he returned 94 punts for 990 yards and 3 touchdowns, and 76 kickoffs for 1,784 yards. Overall, he gained 12,330 all-purpose yards and scored 90 touchdowns. He was named All-NFL five times and to six Pro Bowls.
Kelly led the NFL in rushing for two consecutive seasons (1967–68), and in rushing touchdowns for three (1966–68). He also was a talented punt and kick returner, who averaged 10.5 yards per punt return and 23.5 yards per kick return for his career.
Noted as an exceptionally fine runner on the muddy playing fields of his day (even in the NFL), Kelly gained major yardage in the famed Cleveland trap play up the middle, but was equally devastating on sweeps or as a receiver. His quick-starting ability, along with a sense of balance and knack of evading direct hits by tacklers, kept him relatively injury-free, missing only four games in ten years and never more than one per season.
After his retirement as an active player, he remained in the World Football League as the Philadelphia Bell's offensive backfield coach, joining two other Hall of Famers on that staff, former Green Bay Packers defensive backfield standouts Willie Wood (the first black head coach in pro football history) and Herb Adderley (defensive coordinator).