The Allegheny County Medical Examiner's office said Greenwood died Sunday from undisclosed causes just before noon at UPMC Presbyterian Hospital.
A six-time Pro Bowler and two-time All-Pro, Greenwood played for the Steelers from 1969-81, helping Pittsburgh win an unprecedented four Super Bowls in a six-year span. Greenwood, Joe Greene, Ernie Holmes and Dwight White formed the bedrock of the defense that helped turn a perennial loser into a dynasty.
"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," Chairman Dan Rooney said in a statement. "He will forever be remembered for what he meant to the Steelers both on and off the field."
Greenwood was taken in the 10th round of the 1969 NFL draft - nine rounds after Greene - out of Arkansas A&M (now Arkansas Pine-Bluff). He blossomed into a tenacious pass rusher who used his superior speed to blow past offensive tackles and into the backfield. Though sacks did not become an official statistic until after his retirement, Greenwood posted 73½ during his 13-year career.
Greenwood thrived in the postseason. He sacked Dallas Cowboys quarterback Roger Staubach four times in the 1976 Super Bowl, a 21-17 Pittsburgh victory.
Unlike the quiet Holmes, the intimidating White and the unparalleled Greene, Greenwood was a showman. While recovering from an ankle injury during the 1973 season, Greenwood wore a pair of high top cleats that a friend painted gold. He wore them twice - both Steelers wins - and went back to his usual cleats after the ankle healed. The Steelers lost the ensuing game, and the gold cleats soon returned.
Knee problems forced Greenwood to retire before the 1982 season. His 13 years in Pittsburgh are tied for the third-longest tenure with the team in franchise history. Greenwood remained in Pittsburgh after his retirement, working as an entrepreneur and motivational speaker.
Despite support from his teammates - including Greene - Greenwood has not been enshrined in the Pro Football Hall of Fame. He was a finalist six times, the last coming in 2006.