DOHA, January 25, 2014 (AFP) - With the light fading fast, Garcia clinched the title with a birdie after both players had failed to break the deadlock in their first two attempts on the par five 18th hole.
However when they started all over again for the third time, Ilonen found the bunker and misread his putt from 12 feet to par the hole, while Garcia two-putted for birdie to win his 11th title on the European Tour.
His last two titles in Europe came in 2011 when he won the Andalucia Masters and the Castello Masters.
With Saturday's win Garcia also made amends for dramatically losing out on the title last year in Qatar when Chris Wood's brilliant eagle on the 18th gave the Englishman his first and so far only European Tour victory.
Wood's feat left Garcia tied for second with South African George Coetzee.
"Yeah, it was nice. I think last year, we had a lot of close calls, starting here.
"So very excited to be able to get my hands on this beautiful trophy, beautiful and heavy trophy," said Garcia who is back in the world top 10 after his victory.
"It's a good start to the year. Great to kind of set up my year a little bit, win again on The European Tour. Get more Ryder Cup points. So I couldn't ask for a better week."
Garcia added that it all boils down to good putting.
"As long as you're putting to win, even if you miss, you know that you're still there. What I was a bit more worried about was when Mikko had that eagle chance on the second playoff hole and then he had the chance to win. But it was a tough putt.
"Then on the third hole, hit a really nice 5-iron for my second shot and hit a good putt. I thought it had a chance, it just missed right and he had pretty much the same putt he had for eagle before and still wasn't able to hole it."
Ilonen was left ruing his missed chances in what was his first playoff.
"I was actually expecting that one of us is going to make an eagle. But you know, I hit two poor bunker shots out of that front right trap, one in the regulation and one now in the third playoff hole. I'm not going to blame anything else for my poor shot."
In an action-packed final round several players were in contention for the tile, but Garcia, who was three shots off the pace when play began, shot a bogey-free 65 for an aggregate of 16 under par 272 to put the chasing pack under pressure.
But only Ilonen was up to the task as he sank a birdie from 20 feet on the 18th hole despite finding the bunker to sensationally tie with the Spanish ace.
Denmark's Thorbjørn Olesen was also in with a chance to make the playoff but, but dropped a shot on the 17th after finding the bunker. That left him needing an eagle on the last hole where he only managed a birdie to finish tied third with Rafa Cabrera-Bello on 15 under.
Overnight joint leader Steve Webster missed several easy putts to be tied fifth on 14 under with Thomas Aiken, Coetzee and Alejandro Canizares.
"I had a few chances again, missed one green in regulation, played really good, really happy with my swing," said Webster.
"Just a bit unlucky on the greens. If it had been a little bit of luck, you know, I'd have fancied winning."
Leading scores after the fourth and final round of the 2.5 million dollar Qatar Masters at the par-72 Doha Golf Club on Saturday:
Sergio Garcia beat Mikko Ilonen at the third hole in a playoff to clinch the title.
272 - Sergio Garcia (ESP) 71-67-69-65, Mikko Ilonen (FIN) 68-67-71-66
273 - Thorbjorn Olesen (DEN) 68-69-68-68, Rafa Cabrera-Bello (ESP) 66-65-73-69
274 - Alejando Canizares (ESP) 68-68-73-65, George Coetzee (RSA) 64-69-73-68, Thomas Aiken (RSA) 67-68-70-69, Steve Webster (ENG) 65-69-70-70
275 - Magnus Carlsson (SWE) 69-71-70-65, Dawie Van der Walt (RSA) 65-72-70-68
276 - Romain Wattel (FRA) 67-68-74-67, Paul Lawrie (SCO) 67-70-69-70
277 - Carlos del Moral (ESP) 72-68-69-68, Seve Benson (ENG) 68-71-69-69, Branden Grace (RSA) 67-69-71-70
278 - Dominic Foos (GER) 70-70-70-68, Peter Ulhlein (USA) 70-69-69-70, Simon Dyson (ENG) 68-69-71-70, Darren Fichardt (RSA) 69-70-69-70, Johan Carlsson (SWE) 69-65-72-72, Adrien Saddier (FRA) 70-71-64-73
279 - Peter Hanson (SWE) 69-67-75-68, Tyrell Hatton (ENG) 69-71-71-68, Gaganjeet Bhullar (IND) 67-70-73-69, Matthew Baldwin (ENG) 68-66-74-71, Fabrizio Zanotti (PAR) 69-6968-73