Charlie Appleby's charge was the first Epsom winner to follow up in the 12-furlong Group One since Galileo in 2001, with jockey William Buick saluting the crowd as he passed the post with a length and three-quarters to spare over Mishriff.
Love, winner of last year's 1000 Guineas and Oaks for Aidan O'Brien, was sent off the 13/8 favourite - but had to settle for third, beaten a further length and three-quarters.
Her stablemate Broome slightly fluffed his lines with a tardy start, but he eventually made his way to the front, setting a sound gallop with Adayar racing keenly on his heels for Buick.
Adayar, a 9/4 chance, was clearly travelling well throughout - and when Buick made his move turning for home, the Frankel colt lengthened his stride to assume control and kick for the line.
Mishriff, who raced at the back of the five-runner field, made significant headway in the final couple of furlongs. But Adayar had flown and he galloped out right to the line to give Appleby a first King George win.
"I'm a bit speechless, to be honest," Buick told ITV.
"I'm very fortunate to ride these horses, he's a great horse and he was a great Derby winner, today he was giving me the feeling of endless power.
"It's just a privilege to be able to ride these horses because they don't come around very often."