McDowell, who won the 2010 US Open, overcame current US Open champion Webb Simpson by making par at the first play-off hole after both players had finished the Heritage tournament on nine-under.
On a windy day at Harbour Town Golf Links, McDowell carded a final-round two-under 69 despite a bogey at the last while Webb pulled out all the stops to shoot an even-par 71 as overnight leader Charley Hoffman fell away with a six-over 77.
Given the difficult nature of the conditions, former European Tour Coach Denis Pugh told Sky Sports that both men are indebted to the knowledge and assistance offered by their caddies.
"Both are major champions - they've proved their worth - and they are working hard with good caddies. Ken Convoy, who was caddying for Graeme McDowell today, has done his job to the absolute max, as has Paul Tesori for Webb Simpson.
"We've seen two top professionals with their team doing a really top job and full marks to them both."
Both players found the fairway off the tee on the play-off hole but while McDowell hit the green in regulation, finishing around 16 feet from the pin, Simpson missed just right.
The American came close to holing his third after opting to putt only to see his attempt speed past the hole and travel another six feet after it appeared to be caught by a gust of wind. Simpson then missed the one back.
McDowell acknowledged his opponent's misfortune, saying: "He hit a great putt up there on this playoff hole - he got a terrific gust of wind" and Sky Sports pundit Mark Roe agreed.
"That last putt of Webb Simpson's was almost spooky," he said.
"That ball stopped and then there was a sudden gust from nowhere that blew it on some four feet. Even G-Mac mentioned it there as he shook hands. He said 'it was better than that' and it was.
"Unfortunately Simpson's second putt wasn't. It was a terrible second putt."
England's Luke Donald defied the conditions to shoot a final round 69 that was good enough to earn him a share of third place with Kevin Streelman, who also finished on seven-under 277.
Reflecting on the tournament as a whole, Roe added: "It's a brilliant golf course. It's a tournament I enjoy every year.
"It's been there since 1969, when Arnold Palmer won the first one. It hasn't changed an awful lot since then because it doesn't need to. It has stood the test of time. It's a brilliant golf course and a great event."