The rookie from Northern Ireland, who turned professional six weeks ago after a stellar college career at the University of Alabama-Birmingham, won in just his fourth start on the European Tour. He closed with a 4-under 67 for a 72-hole total of 14-under-par 270.
Trevor Immelman of South Africa posted a 67 for a 13-under-par total and his second runner-up finish of the season.
Immelman looked to be headed to at least a playoff as he and McDowell played the par-4 18th hole knotted at 14-under. But the 22-year-old flew the final green with his approach, chipped up and lipped out a six-foot par putt.
McDowell faced the possibility of a bogey of his own at 18 when his tee shot caught the right fairway bunker for the second day in a row. Though he had to play his second out to the fairway, McDowell stuffed his third shot inside two feet to set up what would prove to be the winning par.
American Jeff Sluman, playing in his first tournament in Europe outside of the British Open, needed a birdie at the 18th to force extra holes. He finished with a bogey, however, for a 69 and a share of third with Norway's Henrik Bjornstad (67) at minus 12.
The last week or so has been a memorable one for McDowell. Last Saturday, the two-time All-American received the Fred Haskins Collegiate Player of the Year Award after a season that saw him win six times in 14 starts for UAB.
Then, two days after turning 23, McDowell fired a 64 in Thursday's opening round to establish a new course record at the par-71 Kungsngen Golf Club. Although he backed up a bit with a 73 in round two - and his record was broken by Per-Ulrik Johansson's 63 on Saturday - McDowell returned to shoot 66-67 on the weekend en route to his maiden title and a two-year exemption on the European Tour.
'I've never been happier,' said McDowell. 'To win in the fashion I did is an incredible feeling. I never dreamed I would do this well in my first six or seven weeks.'
After missing the cut in his professional debut at The Great North Open in June, McDowell made it through all four rounds of the Irish Open in his next start, firing a third-round 65 on his way to a tie for 27th place. He then failed to qualify for the weekend at the European Open in Ireland a month ago.
'I'm just so excited about what it means for me in the short term,' he added. 'I don't have to go to the Qualifying School, I don't need any more invites and I've got somewhere to play for the next two years.'