LONDRES, 19 Oct, 2014 (AFP) - Ilonen clinched the win, in what was the 50th anniversary of the celebrated European PGA Tour tournament, on the 17th green when Stenson, already 2 down, conceded after missing with a long birdie putt.
Setting out shortly after winning their morning semi-finals, the tournament's concluding tie saw some patchy play from both men to start with.
Ilonen though was the steadier and he moved ahead with a birdie putt at the eighth.
The Finn then took back-to-back holes at the 11th and 12th to move three ahead.
Stenson tried hard to claw his way back into it, taking the 13th and 15th holes, but Ilonen kept him at bay by winning the 14th.
The Swede needed to win the last two holes to take the final into a sudden death playoff, but Ilonen hit a better drive and approach close to the pin and Stenson shook hands after missing with his birdie putt.
"The game plan was not to make any mistakes and I didn't make any," said Ilonen, who won the Irish Open this summer.
"I definitely felt really comfortable going into the final and I felt I had a good chance of beating him."
Stenson said that he had exceeded his own expectations just by getting to the final.
"I felt I was running low on energy and I don't know where I found the energy to finish this afternoon," he said.
"I didn't play my best and was unlucky on a couple of holes but all credit to him."
Earlier in the day in the semi-finals Stenson defeated George Coetzee of South Africa at the last while Ilonen saw off the previously unbeaten Joost Luiten of the Netherlands 2 and 1.
Top seed Stenson was two down early on to Coetzee, who only got into the tournament as a late replacement for the injured Thomas Bjorn.
But as he did in the quarter-finals on Saturday against Jonas Blixt, the Ryder Cup player turned up the heat down the back nine going four under for the last six holes.
He levelled the tie with a long putt at the 15th and after Coetzee's putt from the edge of the 18th green missed narrowly, Stenson sunk an 18-footer for birdie and the win.
Ilonen was two up after seven holes against Luiten, but made a mess of the par-three eighth to reach the turn with the slimmest of leads.
Luiten bounced back again to take the lead for the first time at the 13th, but it was back to all-square after the Dutchman three-putted the next.
Ilonen looked to be in trouble at the 15th but a superb chip and putt saw him take the lead and he finished it off at the 17th when another long range effort tipped in from the side.
Luiten took third place in the afternoon by defeating Coetzee at the first extra hole.
Results from the final day of the European Tour World Match Play Championship at the London Golf Club, Kent, England, on Sunday:
Final
Mikko Ilonen (FIN) bt Henrik Stenson (SWE) 3 and 1
Third-place playoff
Joost Luiten (NED) bt George Coetzee (RSA) at first extra hole
Semi-finals
Mikko Ilonen (FIN) bt Joost Luiten (NED) 2 and 1
Henrik Stenson (SWE) bt George Coetzee (RSA) 1up