Mackey's wife notified the team about her husband's death, Baltimore Ravens spokesman Chad Steele said Thursday. No official cause was given.
Mackey played for the Baltimore Colts from 1963-71, and helped the team beat the Dallas Cowboys in the 1971 Super Bowl by catching a pass from Johnny Unitas after it deflected off two other players for a 75-yard touchdown.
He also played for the San Diego Chargers in 1972, and finished his 10-year career with 331 catches for 5,236 yards and 38 touchdowns.
Mackey's efforts after his playing days were just as important as his performance on the field. An NFL labor agreement ratified in 2006 includes the "88 Plan," named for Mackey's number, 88. It provides up to $88,000 a year for nursing care or day care for ex-players with dementia or Alzheimer's disease, or $50,000 for home care.
"John Mackey is still our leader. As the president of the NFLPA, he led the fight for fairness with a brilliance and with ferocious drive," union executive director DeMaurice Smith said.
"His passion continues to define our organization and inspire our players. His unwavering loyalty to our mission and his exemplary courage will never be forgotten."
The health care of former players has become a prominent issue in the negotiations for a new collective bargaining agreement.
"John Mackey was one of the great leaders in NFL history, on and off the field," Commissioner Roger Goodell said. "He was a Hall of Fame player who redefined the tight end position. He was a courageous advocate for his fellow NFL players as head of the NFL Players Association."
Mackey was drafted in 1963 out of Syracuse -- by the NFL's Baltimore Colts in the second round.
His size, speed and ability to catch the ball while also blocking in the running game made him the prototype for future generations of tight ends.
TITANS: Quarterback Kerry Collins is retiring after 16 seasons in the league, his agent announced Thursday. Collins, 38, was facing free agency after his contract with Tennessee expired in March, though he said as recently as last month that he still wanted to play.
In a statement, Collins said his "willingness to commit to the preparation necessary to play another season has waned to a level that I feel is no longer adequate to meet the demands of the position."