In fast fading light at Doha Golf Club, Garcia eventually saw off the Finn with a birdie at the third extra hole to collect the Ã¢Â‚Â¬305,232 winner's cheque and move to second place in The Race to Dubai.
After both men had finished on 16 under par "“ Garcia courtesy of a superb round of 65 and Ilonen with a 66 "“ the duo halved the first two play-off holes with birdie fours.
Third time around, Ilonen's tee shot found the right rough and his approach ended up in the bunker, giving Garcia "“ who had located the heart of the green with his second shot "“ the chance to win it with an eagle three, only for his putt to roll agonisingly past the cup.
Faced with a 20 foot putt to take the play-off into a fourth hole, Ilonen's effort drifted wide to hand Garcia his first European Tour victory since the 2011 AndalucÃƒÂa Masters.
The Spaniard said: "It's been a bit of a wait for my 11th win, I came close a few times last season, including here in Qatar, but didn't quite manage to get the job done. So it's nice to get the win, and great to get more Ryder Cup points on the board.
"I feel like my game's coming together really nicely "“ although I wouldn't have said that after the first round, when I played terribly but shot one of the best rounds of 71 in my career. So to recover from that and grind out a win does an awful lot for my confidence, and the rest of the season excites me."
Despite tasting defeat in the first play-off of his career, Ilonen was still pleased with his week's work.
He said: "I played good golf this week, so I can't be too disappointed. I gave it my best shot, but Sergio was a bit better this time. There's no shame in losing to a great player like him."
Garcia's compatriot Rafa Cabrera-Bello finished in a tie for third place on 15 under par alongside ThorbjÃƒÂ¸rn Olesen after the pair carded respective rounds of 69 and 68.
The leading Australian was Marcus Fraser who finished 54th while Wade Ormsby was 67th.
The European Tour now moves south to Dubai.
Leading Final Round Scores
Garcia 4 4 4
Ilonen 4 4 5
272 S Garcia (Esp); M Ilonen (Fin);
273 R Cabrera-Bello (Esp); T Olesen (Den);
274 G Coetzee (RSA); A CaÃƒÂ±izares (Esp); S Webster (Eng); T Aiken (RSA);
275 D Van Der Walt (RSA); M Carlsson (Swe);
276 P Lawrie (Sco); R Wattel (Fra);
277 B Grace (RSA); C Del Moral (Esp); S Benson (Eng);
278 D Fichardt (RSA); S Dyson (Eng); J Carlsson (Swe); A Saddier (Fra); P Uihlein (USA); D Foos (am) (Ger);
279 M Baldwin (Eng); G Bhullar (Ind); P Hanson (Swe); F Zanotti (Par); R Rock (Eng); T Hatton (Eng);
280 J Quesne (Fra); K Broberg (Swe); S Gallacher (Sco); A Quiros (Esp); H Stenson (Swe);
281 S Kapur (Ind); R Bland (Eng); G Stal (Fra); T Jaidee (Tha);
282 T Lewis (Eng); H Otto (RSA); E Els (RSA); B Koepka (USA); A Wall (Eng); S Hansen (Den);
283 M Siem (Ger); D Willett (Eng); R Fisher (Eng); C Lee (Sco); R Derksen (Ned); R Santos (Por); J Singh (Ind); C Wood (Eng); S Manley (Wal); R Dinwiddie (Eng); S Thornton (Irl);
284 F Calmels (Fra); R Karlsson (Swe); M Fraser (Aus);
285 M Kaymer (Ger); L Donald (Eng); M Foster (Eng); S Kjeldsen (Den);
286 M Hoey (Nir); J Daly (USA);
287 J OlazÃƒÂ¡bal (Esp); E Pepperell (Eng); P McGinley (Irl); J Morrison (Eng);
288 C Doak (Sco); N Elvira (Esp); W Ormsby (Aus);