Phil Mickelson had a 73 and finished 11 shots off the lead in a tie for 38th.
McDowell trailed Simon Khan by two shots until Khan double bogeyed the 12th. McDowell then birdied the next three holes to move ahead and went on to win with a total of 13-under 271 at Loch Lomond.
James Kingston (66) finished second. Richard Green and Miguel-Angel Jimenez shot 69s to tie for third at 274. Khan had a 72 to finish fifth, another stroke back, but secured a spot for the British Open that starts Thursday at Royal Birkdale.
Ernie Els (69) tied for ninth.
Mickelson knows he has work to do before the British Open.
I had a great week this week even though I didnt play the way I wanted to, Mickelson said. I had three good days at Birkdale before this tournament. So this gives me the opportunity to look at some of the strengths I had here as well as the weaknesses, and see if I can put it all together for this week.
Ill spend most of my time on the short game. I turned that around a little today, but overall for me the whole week, it wasnt what I was expecting. I wasnt pleased with the way I chipped or putted this week.
McDowell has now almost certainly clinched a debut appearance in the Ryder Cup at Valhalla in September.
I regard the Ryder Cup as a major, said McDowell, who played at the University of Alabama. Its something I have really wanted to do.
The win put McDowell sixth on the list from which the top 10 qualify.
McDowell holed putts of 10 and 12 feet for birdies on Nos. 6 and 8. He played a bump-and-run shot up close from off the 13th green to set up the first of his run of birdies, drove the green and two-putted the 14th, and then struck a 9-iron to four feet at the next.
Mickelson never got going Sunday. He missed the greens at Nos. 2 and 3 after perfect drives and, despite some superb chips, had two bogeys in his first five holes, including at the fifth for the successive day after acing it Friday.
I was trying to build some momentum, he said. With the putter, Im trying to feel good on it, trying to get better speed and a better line. With chipping, Im trying to be more aggressive.
Els, who won here in 2000 and 2003, wasnt happy with his final round.
I think I could have done a lot better. I played well from tee to green today but the putts dried up, he said. The stroke is not that bad. Its just the speed and the read every now and again.