Only one player who finished ahead of McDowell, Canadian Adam Hadwin, was not already exempt for the July 18-21 tournament in County Antrim in Northern Ireland. Both qualified although the third spot went unclaimed.
"The chase for the Open Championship at Portrush, and getting my spot for that, it's been a journey," McDowell said after ensuring his spot by sinking a 30-foot birdie at the final hole.
"This week, with spots available, people have been reminding me all week. Every hole out there it was like 'hope you get to Portrush, hope you get in the Open' so there was no chance I was going to be able to shove that to the back of my mind.
"It was nice to play decent enough to get it done today."
McDowell's next tournament is this week's U.S. Open at Pebble Beach, the scene of his only major championship victory in 2010, when the tournament was last held at the famous California course.
(Reporting by Andrew Both in Cary, North Carolina; Editing by Greg Stutchbury)
