The 37-year-old Welshman, whose only other European Tour success came in July at the Irish Open on his 255th start, won the £278,172 first prize by a stroke when long-time leader Justin Rose lipped out on the final green from 12 feet. Minutes earlier Donaldson had three-putted for a bogey six, missing from under five feet.
Against a field that had included world top two Rory McIlroy and Tiger Woods – although both missed the halfway cut – he could hardly believe what had happened.
"I played the pro-am on Wednesday and thought the course was too difficult and I had no chance," Donaldson said. "I thought if I could get a decent finish I would be chuffed. To be holding this trophy is just mad."
Rose shared second place with Dane Thorbjorn Olesen, whose 18ft attempt to force a play-off ran just wide.
Donaldson, who earned his Masters spot by climbing into the top 50 by the end of last year, will be around 30th when the new rankings are published today. "It's pretty surreal and I got away with murder there at the last," he added. "The wait was nerve-racking. I thought one of them would hole, if not both."
The six was Donaldson's only dropped shot of the day. He had resumed two behind Rose, but birdied the first, ninth and 11th then sank putts of 18 and 15 feet on the 14th and 15th to go two ahead. His 68 gave him a 14-under-par total of 274, one better than his close friend Rock managed a year ago when he beat McIlroy by one and Woods by two.As for that Masters invitation photo being sent to Rock, Donaldson said: "It's just banter. He was ill in the week and said 'Get my trophy back'." Holding it up he added: "Here you are Rocky."
Rose had led from his opening 67, but closed with a 71. There were three back-nine birdies in that, but also bogeys at the 11th and 17th after he missed both greens. The Englishman, whose runner-up finish takes him back to fourth in the world, said: "It was hard to see the breaks on the front nine, but I pulled it together well and felt I got into a really good, competitive mode.
"I didn't do a lot wrong. I felt like I brought my best stuff on the back nine rather than the front nine, so that's encouraging. I just need to keep chipping away. But it was a long, hard week to end up second."
Donaldson, who spent "four years in the wilderness" after being told his injury troubles were a spinal condition that could end his career, has jumped 26 spots to second behind Scotland's Scott Jamieson in the Race to Dubai.
Stirling's Craig Lee collected his first pay cheque of the year, £25,658, after sharing 17th place on seven-under par, just a shot outside the top 10.
Aberdonian Richie Ramsay, however, who needed to finish in the top five to break into the top 50 in the rankings – fell to a share of 23rd with a round of 75, and the worst score among the top-60 finishes.