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.

At 6 ft 6 in (1.98 m) and 245 pounds (111 kg), 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.

In Super Bowl IX against the Minnesota Vikings in New Orleans, Greenwood batted down two passes from Fran Tarkenton.

Greenwood was known for wearing gold-colored shoes on the football field,[5] to help announcers distinguish him from the higher-profile Joe Greene.

In 2012, the Professional Football Researchers Association named Greenwood to the PRFA Hall of Very Good Class of 2012.