Police were called to Flanagan's home and Baltimore County spokeswoman Elise Armacost said Flanagan's body was found on a trail outside his house. No cause of death was immediately reported.
"It is with deep sadness that I learned of the death of my friend Mike Flanagan earlier this evening," Orioles owner Peter Angelos said. "In over a quarter century with the organization, Flanny became an integral part of the Orioles family, for his accomplishments both on and off the field.
"His loss will be felt deeply and profoundly by all of us with the ballclub and by Orioles fans everywhere who admired him."
Flanagan won the American League Cy Young Award in 1979, going 23-9 with a 3.08 earned run average. He spent 12 seasons with Baltimore before being traded to Toronto in 1987.
He came back to the Orioles as a free agent prior to the 1991 campaign and he retired in 1992.
He had a career record of 167-143 with a 3.90 ERA in 526 games, of which 404 were starts.
In addition to playing for the Orioles, he served for a time as executive vice president of baseball operations and as pitching coach, and also worked as a broadcaster.