Ryder Cup captain Ian Woosnam and Jesper Parnevik were among those who failed to advance at the European qualifier.
Northern Ireland's McDowell, who finished tied for 11th at last year's British Open, will start at Hoylake on July 20 -- along with Englishmen Simon Dyson and Robert Rock, and South Africa's Richard Sterne, who clinched the other three spots Tuesday.
The seven players had made the playoff by shooting a two-round total of 138, six shots off joint leaders Louis Oosthuizen of South Africa and Marco Ruiz of Paraguay, at the Old and New Courses.
A 90-minute rain delay Monday night postponed the playoff, with players returning Tuesday morning to fight it out for the last four qualifying spots.
Darren Fichardt of South Africa double-bogeyed the first hole while Italian Alessandro Tadini had a bogey to reduce the field to five.
McDowell and Sterne sank birdie putts on the third extra hole to guarantee qualification.
Rock was the next man through with a birdie on the 7th before Dyson clinched the last spot on the eighth extra hole by sinking a 6-foot birdie putt after England's James Hepworth missed from 25 feet.
Officials decided to award 18 spots into the Open instead of 15 because of a delay in U.S. qualifying caused by the postponed finish of the rain-hit Booz Allen Classic.
Just 12 spots will be available in the U.S. qualifying on Tuesday, which has been reduced to one round at Congressional because of the delays.