Motley was signed to a contract worth $4,500 a year ($ in dollars). With the Browns, he joined a potent offense led by quarterback Otto Graham, tackle and placekicker Lou Groza and receivers Dante Lavelli and Mac Speedie. He was a force to be reckoned with in the AAFC, and helped the team win every championship in the league's four years of existence between 1946 and 1949. He had a combination of quickness and power – he was listed at 238 pounds – that helped him plow through tacklers. He was also an able pass blocker and played on defense as a linebacker. Motley rushed for an average of 8.2 yards per carry in his first season. His forte was the trap play, a scheme where a defensive lineman was allowed to come across the line of scrimmage unblocked, opening up space for Motley to run. He led the league in rushing in 1948 as the Browns posted a perfect 15–0 record. He was the AAFC's all-time rushing leader when the league folded after the 1949 season and the Browns were absorbed into the more established National Football League (NFL). The Browns had a 47–4–3 overall regular-season win-loss-tie record during the AAFC years as Motley rushed for a total of 3,024 yards.