In 1978, he wrote an article about the Steelers’ violating N.F.L. rules when their players used shoulder pads during a minicamp practice — a revelation that he called Shouldergate and which resulted in the team’s losing a third-round draft pick.
Mr. Clayton left The Press in 1986 for The News Tribune in Tacoma, Wash., where he met his wife, Pat, a sports reporter who covered bowling.
At The News Tribune, he pioneered ways of covering the N.F.L., such as maintaining spreadsheets that tracked every player’s salary after the league introduced salary caps in 1994; calling all 32 teams every Friday to find out who had not attended practice; and contacting every stadium on game days to learn who the inactive players would be.
“John pioneered the granular way in which the league is covered today,” Mr. Sando said.
In addition to his wife, Mr. Clayton is survived his sister, Amy.
His obsession with football began as a child. John Clayton was born on May 11, 1954, in Braddock, Pa., about 10 miles southeast of Pittsburgh. His mother took him to Steelers games, a pastime that only intensified his adoration for the game.
“Of course you can see my body — you can see I didn’t have the ability to compete on the football field,” he told USA Football in 2013. “It just wasn’t there. But I loved the game so much.”
He graduated from Duquesne University in Pittsburgh in 1976 and embarked on his journalism career.
In 1995, he joined ESPN. There, Mr. Clayton’s reporting prominence grew as he starred in weekly radio shows and hosted the “Four Downs” segment with Sean Salisbury, a former N.F.L. quarterback.