Montgomery was drafted by the Philadelphia Eagles in the sixth round of the 1977 NFL Draft. Wearing number 31, Montgomery played eight seasons with Philadelphia, shattering almost all of the Eagles' rushing records and leading the club in rushing six times. Montgomery, who concluded his NFL career with the Detroit Lions in 1985, holds or held seven Philadelphia rushing records, including career attempts (1,465), rushing yards (6,538), attempts in a season (338 in 1979), rushing yards in a season (1,512 in 1979, since broken by LeSean McCoy in 2013), career 100-yard rushing games (26), 100-yard rushing games in a season (8 in 1981), and touchdowns in a game (4). In 1979, Montgomery led the NFL with 2,012 all-purpose yards (rushing, receiving, returns). Over his NFL career, he accumulated 6,789 yards rushing, 2,502 receiving, 814 kickoff return yards, 57 touchdowns (45 rushing, 12 receiving, 1 kickoff return), and two Pro Bowl invitations (1978–79).
Montgomery joined the St. Louis Rams' coaching staff as running backs coach in 1997. In 2004, Montgomery’s coaching ran the gamut, from Pro Bowl running back Marshall Faulk, to young and talented rookie Steven Jackson. Under Montgomery’s leadership, Faulk moved into 12th place on the NFL’s rushing yardage list, and Jackson finished third in the NFL among rookie running backs.
Montgomery and his wife Patti have three children, twins, Briana and Brendan, and a son, Tavian. Montgomery also has a daughter, Sherrita, and a son Derron, who was a wide receiver for the Iowa State Cyclones and is now the Graduate Assistant and Assistant wide receiver coach for the Miami Hurricanes.