He beat Valerii Andriitsev of Ukraine to give the United States their second wrestling gold of the London Games.
Varner, who came third in last year's world championships, sank to his knees at the end of the bout as supporters in the packed arena roared and waved the Stars and Stripes.
"It's awesome," he said. "I expect to win every time I step on the mat. I smiled more than I usually do."
Asked how he would celebrate, Varner added: "I don't know, probably some chocolate milk or something like that."
It was an upbeat end to the Games for the US team after they missed out on gold in the Greco-Roman discipline and in the women's freestyle. Jordan Burroughs won the other gold in the 74kg freestyle.
"Greco had a rough tournament and women had a rough one, but that's how it goes sometimes, that's how the cookie crumbles," Varner said. "We went out there and did our job."
Meanwhile, Azerbaijan's Khetag Gazyumov, who was taken to hospital by ambulance after his heart started racing during a quarter-final defeat, comfortably beat Rustam Iskandari of Tajikistan on the way to bronze.
Gazyumov, 29, left the wrestling mat in a wheelchair after his heart rate soared to a dangerous 260 beats per minute, compared to a typical 60-100.
But he decided to return to the Games when he heard he had earned a place in the bronze medal playoff because he had been beaten by one of the eventual finalists.
"I clenched my fist and decided to do the best I could for my country," he told reporters. "It's strange, but it's sport."