The Portrush man shrugged off the miserable conditions to retain his Alstom Open de France title, carding a closing 67 at Le Golf National, the joint-lowest round on a wet and windy day.
But the former US Open champion also had plenty of help from American Kevin Stadler, who missed from two feet for par on the 18th to force a sudden death play-off.
Stadler had seen his four-shot overnight lead disappear with a front nine of 41, but had battled back well with birdies on the 14th and 16th to pull within one of McDowell, who then carded his only bogey of the day on the 18th after finding heavy rough off the tee.
I would love to win this three times in a row, it's a special golf tournament
That left Stadler needing two pars to force extra holes but after holding his nerve on the 17th, the world number 62 missed his second tap-in of the day to gift McDowell a 10th European Tour title.
McDowell said: "I hit two good shots to 13 to make the putt and thought 'here we go', got a great break at 14, I got a little nudge out of the fringe there, made that put and thought 'this is on now'.
"Everyone here does a great job, it's a great golf tournament and I'm looking forward to coming back next year."
Meanwhile, Northern Ireland's Michael Hoey has secured his place in the Open Championship on Sunday.
Three spots at Hoylake were up for grabs for players, not already exempt, who finished in the top 10 of the French Open.
Sweden's Robert Karlsson finished fourth while Hoey finished seventh, while France's Victor Riu claimed the final berth and Open debut by a single shot despite a double bogey on the 18th in a round of 76.