HILTON HEAD, South Carolina, April 21, 2013 (AFP) - McDowell, the 2010 US Open champion, defeated the reigning US Open title holder Simpson with a par at the first playoff hole.
McDowell had fired an impressive two-under par 69 at wind-swept Harbour Town Golf Links, marred only by a bogey at the final hole that left him on nine-under 275.
He was joined there by Simpson, who carded an even-par 71 that included three birdies and three bogeys to force the playoff.
England's Luke Donald also posted a 69 to share third place with American Kevin Streelman, who carded a 72 for seven-under 277.
Overnight leader Charley Hoffman struggled in the windy weather and his six-over 77 saw him fall into a tie for sixth place on five-under 279.
McDowell notched his first US PGA Tour of the season.
When they returned to the 18th for the playoff, McDowell shrugged off his regulation bogey at the hole and landed his approach shot some 15 feet from the pin, and then two-putted for par.
Simpson, who missed the green, nearly holed his next shot with his putter from the green, only to see it caught by the wind and roll six feet past.
After McDowell had safely parred, Simpson missed his par attempt coming back.
"I never thought a putt was in more than that one," Simpson said of his first effort. "With a foot to go, it was in."
McDowell thought the wind was a factor in Simpson's miss.
"I'm disappointed for Webb," he said. "He hit a great putt up there on this playoff hole -- he got a terrific gust of wind."
But McDowell was pleased with his first PGA Tour win of the season -- a first for a European on the US circuit in 2013.
"I played nicely all week," he said. "I played great Friday. The putter was cold most of the week and I really got it going today."
Leading final-round scores here Sunday in the $5.8 million US PGA Tour Heritage tournament (x-won in playoff; USA unless noted, par-71)
275 - x-Graeme McDowell (NIR) 71-67-68-69, Webb Simpson 68-71-65-71
277 - Luke Donald (ENG) 69-68-71-69, Kevin Streelman 66-70-69-72
278 - Jerry Kelly 69-72-66-71
279 - Russell Henley 73-70-67-69, Chris Stroud 70-70-69-70, Charley Hoffman 66-70-66-77
281 - Mark Wilson 69-75-67-70, Rory Sabbatini (RSA) 69-69-72-71, Trevor Immelman (RSA) 72-72-66-71, Marc Leishman (AUS) 67-71-71-72, Richard H. Lee 68-71-69-73, Jordan Spieth 70-69-69-73, Billy Horschel 71-68-68-74, Camilo Villegas (COL) 68-71-68-74, Brendon de Jonge (ZIM) 70-69-67-75
282 - K.J. Choi (KOR) 70-71-71-70, Ken Duke 70-70-71-71, Johnson Wagner 67-71-71-73, Pat Perez 68-70-70-74, Steve LeBrun 68-68-71-75, Justin Hicks 69-70-68-75
283 - Scott Langley 71-69-72-71, Stewart Cink 70-69-73-71, Robert Garrigus 70-71-70-72, Darron Stiles 70-69-71-73, Bill Haas 68-69-70-76, Tim Clark (RSA) 68-71-68-76