McNeil played in 12 NFL seasons for the Jets from 1981 to 1992. During the mid to late 1980s he was a member of the Jets' "Two Headed Monster" backfield along with teammate Johnny Hector, a tandem that ranked among the league's elite. When he retired he was the Jets all-time leading rusher with 8,074 yards; he was surpassed by Curtis Martin and currently ranks second in Jets team history. In 1982, McNeil led the NFL in rushing with 786 yards. He was the first Jet to the lead the league in rushing. He is one of a few running backs in NFL history to average 4.0 yards per carry in every season he played.
From 1990 to 1992 McNeil was the lead plaintiff in a case won by jury verdict that struck down the NFL's Plan B free agency system, under which teams could protect 37 players. McNeil and the seven other plaintiffs were among the protected players listed by their teams. The system was deemed too restrictive and a violation of antitrust laws. However, Freeman was not one of the four plaintiffs awarded damages. The suit is considered a major step in the achievement of free agency rights by the NFL Players Association.