It was a birdie after a chip in from the fringes at the last hole which earned him a place at the top after he held the lead briefly on his back nine as fortunes fluctuated too frequently on the third day.
Gurgaon: For the second week running, Shubhankar Sharma was unable to cash in on his pole position to close out a title win.
One can not help feeling for Emiliano Grillo, who had two great days on Thursday and Friday but then came a cropper on the weekend.
Among other Indians, Ajeetesh Sandhu, who was best-placed Indian after the first round, was second best at 26th after he carded two-over 74 on day 3.
Sharma who had started the day at the number one spot failed to take the advantage of his lead and faltered again in the final round as he repeated his final round slip up from the last week's Mexico World Golf Championship. On the final day he kept his nose ahead of challengers like Johnston, Kim Sihwan, Pablo Larrazabal and Matt Schwab with a solid front nine of four-under.
Englishman Wallace retained his share of the lead with a four-under 68, but a bogey on the 16th proved costly as Johnston signed for a brilliant 66 for the second time this week to force an extra hole with the pair tied on 11 under for the tournament.
Johnston's birdie effort from the left edge of the green looked destined for the hole, but spun around it and stayed out, leaving Wallace two putts for the title.
He parred the next to make the turn at two-under but he was still one stroke behind Grillo, who picked up a birdie at the ninth hole. The Indian found the bunker at the 10th and 13th holes.
Despite taking a free drop, he ended up taking six shots to complete the hole.
Sharma, 21, also led into the final round of last week's WGC event in Mexico but closed six shots behind eventual victor Phil Mickelson.
Argentine Emiliano Grillo was then five under, with Race to Dubai leader Sharma and Gallacher a shot further back in a tie for seventh. The double-bogey on the fifth compounded my problems.