In 1946, Marion Motley signed with the Cleveland Browns, joining his teammate Bill Willis, and reentered professional football. Despite facing significant racial hostility, he emerged as one of the most formidable athletes of his time. During his eight-season tenure with Cleveland, Motley was recognized as an indomitable power runner and an elite pass protector, functioning as the key physical force within Paul Brown's offensive success.

Motley was constructed with the physique of a contemporary interior lineman, yet he played in a manner reminiscent of a backfield player from the 1940s. He exhibited a rare blend of raw power, lateral agility, and explosive speed. Employing the trap-draw formations pioneered by Paul Brown, Motley overwhelmed interior defensive lines through vigorous leg drive and second-level acceleration. In addition to his running capabilities, his blitz pickup was renowned for its physicality, effectively neutralizing rushing linebackers and providing Otto Graham with the unobstructed pocket necessary to execute down-the-seam passes.

Motley assumed leadership of the All-America Football Conference (AAFC) immediately upon entering the field. He secured the 1948 AAFC rushing title with 964 yards, significantly contributing to Cleveland's undefeated season. Throughout the Browns' four-year participation in the AAFC, Motley led the league in career rushing yards with a total of 3,024 yards, while Cleveland secured all four AAFC Championships.

When critics questioned whether the Browns could compete against the established NFL in 1950, Motley responded by securing the NFL rushing title with 810 yards on an impressive 5.8 yards per carry. His notable single-game performance occurred that season against Pittsburgh, where he rushed for 188 yards on only 11 carries (17.1 YPC). Cleveland concluded its inaugural NFL season by winning the 1950 NFL Championship, thereby establishing their position as the preeminent football team of the period.

Motley received his only Pro Bowl selection in 1950 and was honored with two First-Team All-Pro distinctions in 1950 and 1951. He concluded his tenure with Cleveland after the 1953 season, having accumulated 4,712 rushing yards and 31 rushing touchdowns, with an impressive average of 5.7 yards per carry—a record that continues to rank among the highest in the history of professional football.

In 1968, Motley became the second African American in the Pro Football Hall of Fame. In 2010, the Browns honored his groundbreaking career by naming him to the inaugural Cleveland Browns Ring of Honor.