The 7-2 favorite, ridden by 28-year-old amateur jockey Sam Waley-Cohen and trained by Nicky Henderson, took the lead for the first time when jumping the second-to-last fence and pulled away from former winners Denman (8-1) and Kauto Star (5-1) in the sprint to the line.
Aged 6, Long Run became the youngest winner of the prestigious race since Mill House in 1963.
“This is beyond my wildest dreams,” Waley-Cohen said.
“I didn't think we were going to get there, but he picked up and I think he still had a bit left.
“He's a whole lot better than I thought he was. That's a very special feeling and he's a very special horse.” Denman finished second, the 11-year-old maintaining his record of never having finished outside the top two in seven starts in Cheltenham.
Two-time winner Kauto Star, who was denied a fifth straight King George VI title by Long Run in January, finished third ahead of What a Friend, the 25-1 shot owned by Manchester United manager Alex Ferguson.
“I can't think of a better race than that,” Ferguson said.
Paul Nicholls, the trainer of both Kauto Star and 2008 winner Denman, said his 11-year-old horses couldn't have done any more.
“They've run absolutely amazing races. I thought they had it between them at one stage, but Long Run was always going well just behind,” Nicholls said.
“I always thought Long Run was the best horse in the race but I could not be more proud of my horses.” Henderson said Waley-Cohen gave Long Run “a beautiful ride” to win British steeplechasing's biggest prize.
“I thought he was going to need a bit more daylight coming up the hill (after the final fence) but Sam was very confident and he was very professional,” said Henderson.
