Both players finished the regulation 72 holes on 16-under-par 272, one ahead of Spain's Rafael Cabrera-Bello (69) and Dane Thorbjorn Olesen (68).
Garcia (65) and Ilonen (66) then carded matching birdie fours at the 18th in the first two holes of the playoff, the second from the Spaniard coming after a magical bunker shot to tap-in range just when it looked as though the title may be heading to Finland.
The pair kept playing the 18th and at the third time of asking Ilonen could only manage a par after finding rough off the tee and a bunker with his second shot while Garcia hit the green in two and safely two-putted for yet another four.
The 34-year-old broke into a wide smile before embracing his delighted caddie as Garcia won on the tour for the first time since landing the Andalucia Masters crown in October 2011.
The Ryder Cup stalwart's victory at the Doha Golf Club made up for narrowly missing out on the grand Mother of Pearl Trophy 12 months ago when Britain's Chris Wood eagled the final hole.
"This feels good," Garcia told Sky Sports television after following up his Asian Tour triumph at the Thailand Championship last month. "I started the day thinking that if could shoot six or seven under maybe I would have a chance.
"I had a very similar putt to last year," he said of a 10-foot birdie attempt that slid wide of the cup at the 72nd hole.
"I hit a good one but unfortunately it didn't break. Then the up and down from the bunker was key at the second hole of the playoff and that helped me win."
Former world number two Garcia, who now has 11 European Tour victories to his name, said he felt no added pressure during the sudden-death holes.
"I almost feel less pressure in playoffs because you feel like you've had a great week already," he explained.
"Obviously you're trying to win but by that stage it's all a bit of a bonus because you've already had an amazing week. I was more nervous over my tee shot at the 72nd hole than I was over any of them in the playoff."
World number 11 Garcia's victory, the second in a row by a Spaniard after Pablo Larrazabal won the Abu Dhabi Championship on Sunday, earned him a first prize of $416,000 and will lift him back into the top-10 of the rankings.
The final event of the tour's three-week Desert Swing will be the Dubai Classic, featuring world number one Tiger Woods and seventh-ranked Rory McIlroy, that starts on Thursday.
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