Greenwood was selected in the tenth round of the 1969 NFL/AFL draft by the Pittsburgh Steelers, who had finished at 2–11–1 the previous season, and replaced head coach Bill Austin with Chuck Noll. In , he became the starting left defensive end. One of the four members of Pittsburgh's famous Steel Curtain, he remained there until retirement in . At and , Greenwood was a six-time Pro Bowl player and was named to NFL All-Pro teams in 1974 and 1975, and was All-AFC five times. He also led the Steelers six times in sacks with a career total of 73½ (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.