John Mackey, a Hall of Fame tight end with the Baltimore Colts, who was a pioneering union leader for NFL players and whose later struggles with dementia highlighted the long-term effects of brain damage among football players, died July 6 at Keswick Multi-Care Center in Baltimore. He was 69.

The cause of death for Mr. Mackey was not released, but dementia was diagnosed in 2001.

Mr. Mackey began to redefine how the position of tight end was played in his first game with the Colts in 1963, when he scored a touchdown on a 32-yard pass from quarterback Johnny Unitas.

Until then, tight end had been primarily a position for blocking and short-yardage passes. At 6 feet 2 inches tall and 230 pounds, Mr. Mackey was a powerful blocker, but his outstanding speed brought an extra dimension to the offense by making the tight end a constant threat for a long touchdown pass. Of his seven touchdowns in 1963, all came on plays of at least 27 yards, with three of more than 50 yards.

The most dramatic catch of his 10-year career came in the 1971 Super Bowl against the Dallas Cowboys. In the second quarter, a pass from Unitas slipped through the fingertips of receiver Eddie Hinton, then was tipped by Dallas defensive back Mel Renfro into the hands of Mr. Mackey. He clutched the ball and ran for a 75-yard touchdown, sparking the Colts to a 16-13 victory.

As polished off the field as on, Mr. Mackey regularly appeared on Baltimore radio and television programs and once acted as host at a symphony concert. He became a leader in the NFL Players Association and, in 1970, became its first elected president.

That year, he led a players strike that won millions of dollars in pensions and benefits for the players. He filed an antitrust suit against the NFL that temporarily allowed players to negotiate as free agents with team owners. (In 1977, the players gave up that right as part of a new labor agreement.)

“People will never fully understand the impact he had on negotiations between players and owners,” former Colts star Lenny Moore told the Baltimore Sun. “John unlocked those gates — no, he knocked the doors down.”

In retirement, Mr. Mackey settled in Los Angeles and became a successful businessman. Over time, his wife, Sylvia, began to notice that he was becoming uncharacteristically lethargic and forgetful.

Mr. Mackey received a diagnosis of frontal lobe dementia in 2001. Although it was never definitively linked to football, he withstood countless blows to the head and once ran into a goalpost at full speed.

He and his family moved back to Baltimore in 2002 as his condition grew worse. His wife became a flight attendant at age 56 to obtain health insurance and pay bills.

Mr. Mackey had to stop going to autograph shows after he refused to take off his Super Bowl and Hall of Fame rings at an airport checkpoint. He charged through like, well, a runaway tight end before he was tackled by four security officers.

He was banned from several restaurants after repeatedly walking up to tables to ask if anyone wanted his autograph.

He answered questions such as “How are you?” by turning them into football plays: “I’m on offense. I’m blocking.”

When Mr. Mackey balked at brushing his teeth, his wife placed an official-looking memorandum in the bathroom. Purporting to be from the NFL, the memo required all players to brush their teeth and take a shower.

In 2006, Sylvia Mackey wrote to NFL Commissioner Paul Tagliabue, describing her husband’s physical condition and the financial ruin it was causing her family. At the time, Mr. Mackey was drawing an NFL pension of a little more than $2,000 a month.

A year later, the league and the NFL Players Association jointly created the 88 Plan, named after Mr. Mackey’s jersey number. It allowed families to receive up to $88,000 a year to care for a disabled player.

In the first 18 months of the program, the families of more than 100 former players had signed up, bringing to light the hidden problem of long-term brain damage among onetime NFL stars.

“A lot of the retired players just keep suffering in silence,” Bruce Laird, a former Colts defensive back who is a leader in the effort to gain greater benefits for aging football players, told The Washington Post in 2008. “Sylvia [Mackey] fought and won. But it was Sylvia. Not the union.”

John Kevin Mackey was born Sept. 24, 1941, in Queens, N.Y., and grew up in the Long Island community of Hempstead, N.Y., where his father was a Baptist minister.

Mr. Mackey was a graduate of Syracuse University, where his roommate was Ernie Davis, who in 1961 became the first black player to win college football’s highest honor, the Heisman Trophy. Davis, who died of leukemia at 23, wore No. 44 at Syracuse. Mr. Mackey wore No. 88 because it was the double of his roommate’s jersey number.

In 1963, Mr. Mackey played on a college all-star team that defeated the NFL champion Green Bay Packers.

With the Colts, Mr. Mackey played in two Super Bowls and one NFL championship game. He spent his final season with the San Diego Chargers in 1972. During his career, he caught 331 passes for 38 touchdowns, including 13 of at least 50 yards.

Survivors include his wife of 47 years, Sylvia Cole Mackey of Baltimore; three children, John Kevin Mackey of Atlanta, Lisa Hazel of Bowie and Laura Nattans of Baltimore; and six grandchildren.

When Mr. Mackey was elected to the Pro Football Hall of Fame in 1992, he was only the second tight end to be inducted, after Mike Ditka of the Chicago Bears. He refused to go to Indianapolis, where the Colts had moved in 1984, to accept his Hall of Fame ring.

“I will do it in Baltimore,” he said. “That is where I played.”