McCoy died in the rural West Texas town of Menard after battling a long illness, said Wayne Vincent, president of the Austin Police Association.
Following more than an hour of mayhem on the UT campus -- Whitman was so terrifyingly accurate with his high-powered rifle that he shot people as far as 500 yards away -- McCoy and a small group of others made their way to an observation deck atop the 28-story tower. McCoy fired twice from his 12-gauge shotgun, shooting Whitman in the face. Officer Ramiro Martinez also fired on Whitman.
In an interview with the Austin American-Statesman in 2011, McCoy insisted he didn't want Whitman mentioned in his obituary and was reluctant to even utter the sniper's name. He wondered whether he could have saved more lives if he had reached the observation deck sooner.
"I don't want a headline that reads `UT Tower Hero Dead,"' McCoy told the newspaper. "If the word hero has to be used, then there were many heroes. I was just one of them."
Whitman, 25, was a Texas student and former Marine when he opened fire just before noon Aug. 1, 1966. Three decades later, a 17th death would be attributed to him in 2001 when a Fort Worth man died of injuries he suffered when he was shot that day.
Authorities later determined Whitman also killed his wife and mother in the hours before he went to the tower.
McCoy was 26 when he was riding in the squad car and made his way toward the campus after reports of a gunman.
"He was a good police officer who answered the call," Vincent said. "He and the rest of the guys who went into that tower had no idea if they were going to survive or not, and they went. That's big."
Whitman's rampage was the deadliest school shooting in U.S. history until the 2007 Virginia Tech massacre.
Funeral services for McCoy have not yet been announced.