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

Labour, Plaid Cymru and the Liberal Democrats opposed new thresholds on when strikes can take place.
The vote against Conservative ministers' Trade Union Bill is not legally binding.
The UK and Welsh governments disagree over whether the assembly's permission is needed before the new rules apply to public sector workers in Wales.
The bill would require at least half of union members to take part in a ballot before a strike can happen.
In public services such as health, education and the fire service, industrial action would need at least 40% of eligible union members to vote in favour.
The Conservatives say the bill should apply to the whole of Britain, but Welsh government ministers say the assembly's consent is needed.
On Tuesday, AMs withheld their approval of the bill by voting down a motion in the Senedd chamber.
Labour has said it will try to overturn the bill if the UK government ignores the assembly' vote and pushes ahead.
If returned to power at May's assembly election, Labour would introduce its own legislation to repeal the bill.
Public Services Minister Leighton Andrews said: "In Wales we have a good record of resolving disputes.
"There was no junior doctors' strike in Wales, there was in England.
"Fire fighters took industrial action in England over pensions, they were not doing so in Wales.
"In contrast, we believe this bill, the Trade Union Bill, will lead to a confrontational relationship between employers and the workforce."
Welsh Conservative leader Andrew RT Davies said he supported the role of trade unions in standing up for members.
But, referring to the general secretary of the Unite union, he said: "Where I will not stand up and support is for Len McCluskey, or his mob, to use you lot [Labour AMs] to live their ambitions."
He added: "Where there is a discrepancy I would suggest is where general secretaries of unions choose to live their political dreams on the back of their members' subs, and that clearly is not acceptable."

The 23-year-old, who can also play in defence, made 56 league appearances for the Spireites.
He has signed too late to feature in Saturday's home game against Walsall.
Boss Kenny Jackett told the club website: "We have brought in a bright and energetic young player. He has a very good left-foot and adds versatility to our squad."
Find all the latest football transfers on our dedicated page.
Graeme McDowell began the defence of his World Match Play title by beating France's Alexander Levy 3&2 in Wednesday's opening match.
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Assembly members have voted against UK government proposals to restrict strike ballots in the public sector.
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Portsmouth have signed Chesterfield midfielder Dion Donohue for an undisclosed fee on a two-year deal.