The 23-year-old McDowell carded a final-round 5-under-par 67 at Kungsagen -- the course he recently agreed to represent -- to claim victory in just his fourth European Tour start.
The 12th first-time winner on the tour this season, McDowell won despite driving his tee shot against the lip of a fairway bunker at the 18th hole. He played out sideways but put his third shot within two feet of the hole before making par.
Immelman flew the green with his approach and was unable to get up-and-down for par, leaving him a stroke off McDowell's winning total of 14-under 270.
PGA Tour veteran Jeff Sluman and Norway's Henrik Bjornstad tied for third at 12-under 272. Sluman needed a birdie at the 18th to force a playoff but bogeyed.
McDowell overcame a handful of mistakes to win his first title. A former Walker Cup team member who played at the University of Alabama, McDowell shanked a bunker shot at the 15th hole, sending his ball into a lake that surrounds the green. After a drop, McDowell pitched to six feet and saved bogey. He made a 12-foot birdie putt at the 16th.
Although he will represent Kungsagen, McDowell had not seen the course until Tuesday. Then he recorded a course-record 64 on Thursday.
McDowell is no stranger to winning events. He claimed six of the 12 he entered at Alabama and recorded a 69.60 scoring average, better than the previous NCAA record established by Tiger Woods and Luke Donald.