Johnston, who entered the day in joint fourth, sank six birdies in a bogey-free 66 to force a play-off with Wallace.
Wallace attacked the first extra hole, the par-five 18th, and birdied to win the tournament after Johnston took a more conservative approach to finish on par.
"I played great," Wallace said. "Ever since being three over through eight at the start of the week I've played some of the best golf of my life and to do it in that style at the end there capped it off.
"There are some great champions. SSP (Chawrasia) held it for a couple of years there and I am so happy to get my name on the trophy.
"I asked on 15 what the score was. I didn't hit many great iron shots on the back nine but I knew where I stood. I like to know what I have to do," added the 27-year-old.
Sharma, 21, had another final round to forget following last week's collapse at the WGC-Mexico Championship, as the Indian went three-over 75 to finish at four-under along with Scotland's Stephen Gallacher.
Kim Sihwan sealed third position on eight-under after carding an impressive 68 in the final round while Spaniard Pablo Larrazabal and Austrian Matthias Schwab were a shot behind at joint-fourth.
(Reporting by Aditi Prakash in Bengaluru; editing by Amlan Chakraborty)
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