McDowell was joined atop the leaderboard by Spain's Jose Manuel Lara and Australian Nick O'Hern. Englishman Nick Dougherty shares fourth place at 4-under-par 68 with Raphael Jacquelin, Jarrod Moseley, Phillip Price, Raymond Russell and Patrik Sjoland.
There are four members of the winning European Ryder Cup team in the field this week. Through the opening round, Colin Montgomerie and David Howell lead the way for that group at 1-under-par 71. Padraig Harrington had a 72. The only Ryder Cup player to have a rough round was Ian Poulter at 77.
McDowell got off to a rough start as he double bogeyed the first but he battled back to birdie the par-4 third. After six straight pars, the Ulsterman turned it on when he reached the 10th tee on the Duke's Course at Woburn Golf and Country Club.
The 25-year-old dropped in a birdie at the 10th to get back to even par for his round. McDowell, a two-time winner on the European Tour, poured in a birdie on 11 to make it two in a row.
He ran off three consecutive birdies from the par-4 14th to move to 4 under. McDowell closed with a birdie at the last to grab a piece of the lead.
'It was such a relaxed atmosphere starting out today. I was chatting with Colin Montgomerie about the Ryder Cup,' said McDowell, who played alongside Montgomerie. 'The first five or six holes were so relaxed. It was kind of tough to get into my rhythm. We really started to have a good time coming into the last ten or 11 holes. It was nice to come back after a week a off.'
Lara had a hot start as he birdied three of his first five holes. He tripped to a bogey at the seventh, but came right back with a birdie on No. 8. The Spaniard moved to 4 under when he drained a birdie at the par-5 10th.
Lara faltered to bogeys at the 13th and 15th to slide back to minus-2. He ran home a birdie at the 16th and came right back to birdie 17. The 27-year-old made it three straight to end his round as he birdied the par-5 closing hole.
'I have been playing a little bit up-and-down this year,' Lara said. 'The last few weeks I have not played so well. Maybe I am a little bit tired as this is my 27th tournament of the year. With the Volvo Masters this year I am probably going to play 31, which is too much.'
O'Hern started on the back nine and opened with a birdie on No. 10. He converted a birdie on the 14th, but tripped to a bogey at 15. He finished his opening nine at minus-2 as he dropped in a birdie at the 18th.
The left-hander drained a birdie at the fourth. O'Hern then picked up consecutive birdies from the seventh to join McDowell and Lara atop the leaderboard.
Jamie Elson, Klas Eriksson, Mark Foster, Stuart Little, Jean-Francois Lucquin, Roger Chapman and Henrik Stenson share 10th place at 3-under-par 69.