Steelers Chairman Dan Rooney and President Art Rooney II said in a statement released after the Steelers lost to Minnesota in a game in London that Greenwood -- a member of the lauded Steel Curtain defense -- had died Sunday afternoon.
No cause of death was given, but the Pittsburgh Post-Gazette reported he had been hospitalized for several days.
Greenwood, who played for the Steelers from 1969-81 and was a member of four Super Bowl-winning squads, finished his career with an unofficial total of 73 1/2 sacks over 170 games.
"L.C. Greenwood was one of the all-time great Steelers and one of the leaders of the legendary 'Steel Curtain' defense in the 1970s," the Rooneys said.
"L.C. was one of the most beloved Steelers during the most successful period in team history and he will be missed by the entire organization. He will be forever remembered for what he meant to the Steelers both on and off the field."
The lineman is probably best remembered for his four sacks of Dallas Cowboys quarterback Roger Staubach in Super Bowl X.
Greenwood was named to the Steelers' all-time team in 2007.