Ridden by Olivier Peslier, the 4-1 second favorite, on his debut in Group One company, raced clear at the 400 meter mark and came right away from Irish Derby winner Cape Blanco (9-2).
Harbinger beat the course record time by just under half a second in an electrifying display which prompted one bookmaker to slash the four-year-old from 8-1 to even money favorite for the Prix de L'Arc de Triomphe in October.
It was a fifth success in the King George for trainer Michael Stoute who first enjoyed a winner in the big race back in 1981 with the ill-fated Shergar.
Harbinger, a still improving horse, is owned by the Highclere Thoroughbred Racing syndicate who enjoyed success in the Epsom Derby with Motivator in 2005.
Stoute told BBC TV: "Olivier was just cantering. He traveled so well and he was looking good from a long way out." Workforce, who won last month's Epsom Derby in record breaking-time, started a warm 8-11 favorite but struggled to go Saturday's pace as the six-strong field moved into the straight.
Stable jockey Ryan Moore, who opted to ride Workforce instead of Harbinger, blamed the firm ground for his colt's defeat.
