McDowell overturned an eight-shot deficit thanks to a brilliant closing 67 despite the wet and windy conditions at Le Golf National, although he also had plenty of help from American Kevin Stadler.
Stadler’s four-shot overnight lead disappeared with a front nine of 41, but he battled back well with birdies on the 14th and 16th to pull within one of McDowell, who had moved three ahead with his fifth birdie of the day on the 16th.
With McDowell then dropping his only shot of the day on the 18th after finding heavy rough off the tee, Stadler needed to par the 18th to force a play-off but missed from two feet just as McDowell left the recording area to prepare for extra holes.
Stadler’s closing 76 left him in a tie for second with Thailand’s Thongchai Jaidee, who dropped five shots in the first four holes but played the remainder in four under for a 72.
“I’m very surprised,” McDowell said after claiming his tenth European Tour title. “Midway through my second glass of red wine last night, when I was very disappointed with my back-nine performance on Saturday, I really didn’t think I’d be, a) standing here with an opportunity to be in a play-off; and, b) with a trophy in my hands.
“I feel very fortunate. Kevin Stadler is a great, great player. I literally gave him that putt on the last green. I didn’t expect him to miss that.
“It’s not really the way you like to win. I was ready to go for the play-off, but I’ll take it and run. I really needed this victory. It’s a special one for me. It’s pretty timely. There’s a busy summer ahead and this is a really good kick-start to the summer.”
McDowell has deliberately played a lighter schedule this year to leave himself fresher for the second half of the season.
“It’s a good decision now, but I have to say myself and my team were starting to question it,” the former US Open champion added on Sky Sports 4. “But my body has probably never been in as good a shape at this point in the season coming into two major championships, the FedEx Cup play-offs and, hopefully, the Ryder Cup.
“I’m very proud to defend my title and this is the boost I needed on many fronts, especially the Ryder Cup [the win takes him to tenth on the European points list].”
McDowell, who held the outright lead after the first round of the Open the last time it was held at Hoylake in 2006, added: “I was at Hoylake a couple of weeks ago.
“I think it’s my type of set-up. You really have to find fairways off the tee and there are big flat greens where you can hole some putts. This is a nice confidence booster and I will be back at Hoylake next Sunday ready to go.”
Stadler found himself five clear of the field when playing partner Jaidee got off to a terrible start and local favourite Victor Riu followed a birdie on the first with a double bogey on the second.
But the 34-year-old then missed from 18 inches for par on the fourth and four feet on the sixth, before running up a double-bogey six on the next where his ball was only found in the deep rough because an on-course television commentator stood on it. A poor chip from left of the eighth green led to another bogey and two more early in the back nine looked to have ended his chances as McDowell picked up shots on the 13th, 14th and 16th.