Greenwood was drafted by the Pittsburgh Steelers in 1969 in the 10th round. In 1971, he became the starting left defensive end. One of the four members of Pittsburgh's famous Steel Curtain, he would remain there until retirement in 1981. Greenwood, who was 6 feet 6 inches tall and weighed 245 pounds, was a six-time Pro Bowl player and was named to NFL All-Pro teams in 1974 and 1975. He was named All-AFC five times. He also led the Steelers six times in sacks with a career total of 73½ (sacks were an unofficial stat at the time). According to records kept by the Steelers, Greenwood's highest single-season sack total was 11, which he attained in 1974. He further had 14 fumble recoveries in his career, including five in 1971, which tied for the NFL lead. He had 4.7 in the 40-yard dash, and that speed allowed him to dominate his position.
Greenwood was known for wearing gold-colored shoes on the football field to help announcers distinguish him from the higher-profile Joe Greene. Greenwood was called “Hollywood Bags” because of his desire to become an actor after retiring from football. He was a finalist in the 2005 Pro Football Hall of Fame voting but did not get elected. He was again a finalist in 2006, but was not elected. Greenwood has stated that while he would be honored if he were to be inducted into the Hall of Fame, he would not be upset if he were not elected, feeling that the Steelers already in the Hall (in particular, "Mean Joe" Greene) represent the entire team's accomplishments.
In 1991, Greenwood was named to the Super Bowl Silver Anniversary Team and in 2007 he was named to the Steelers All-Time team.