Four birdies in the first six holes set the Northern Irishman on his way at the London Golf Club near Maidstone.
England's Paul Casey beat Wales' Jamie Donaldson, while Ireland's Shane Lowry beat Scotland's Stephen Gallacher 3&2.
Other winners on day one were Joost Luiten, Thongchai Jaidee, Jonas Blixt and Victor Dubuisson.
Having successfully partnered McDowell to two foursome wins in Europe's recent Ryder Cup success, Frenchman Dubuisson matched McDowell's run of four birdies in the opening six holes as he overcame Spaniard Pablo Larrazabal 3&2.
Another Ryder Cup winner, Sweden's Henrik Stenson, halved with South African George Coetzee.
The 16 competitors, divided into four groups of four, play three group matches each on Wednesday, Thursday and Friday before Saturday's quarter-finals and Sunday's semi-finals and final.
"There are no easy matches. You can't take anyone lightly, especially a guy who won last week, so I'm very happy to get the victory," said McDowell.
"It's important to control your own destiny in the group stages - you don't want to be relying on other people."
Full World Match Play results and tables
Graeme McDowell began the defence of his World Match Play title by beating France's Alexander Levy 3&2 in Wednesday's opening match.