Wallace (69-70-70-68) and Johnston (72-66-73-66) were tied at the top after regulation 72-hole play with matching totals of 11-under 277, forcing a playoff at the DLF Golf & Country Club.
Wallace, who went into the fourth round with a one-shot lead with 21-year-old Shubhankar, got a birdie in the playoff played at the 18th hole, as Johnston could only make a par effort.
Shubhankar started the fourth round with two birdies through four holes. However, his tournament unravelled after double bogeys at the fifth and seventh holes.
The Asian Tour and European Tour leader carded his worst round to finish with a total of four-under 284.
"It was a disappointing day for me. I had a few bad swings at the start that put me off my rhythm. The double-bogey on the fifth compounded my problems. Thereafter, it was all about trying to catch up with the lead," he said.
"I had a few birdies today but there were just too many bogeys for my liking. But these things happen. Regardless of the result I'm quite happy that after being five-over through the first nine holes of the tournament I managed to finish with a four-under total."
The silver lining for Shubhankar, who picked up a cheque of $48,125 for his effort this week, was that he continues to lead the two Tours.
Sihwan Kim of the United States made a charge on the last day of the tournament, but he was not able to keep up with the relentless pace set by the leaders. The 2017 Asian Tour Qualifying School graduate shot five birdies and a bogey for 68 to finish the tournament in third place.
The 27-year-old Wallace, however, stole the show. He started magnificently by scoring four birdies before making the turn. He made another birdie on the 10th and a bogey on the 16th before signing for 68.
Johnston put in a bogey-free performance and forced a playoff with Wallace after carding a 66, the day's lowest score.
Wallace, who was ranked 152nd going into the this tournament, will move into the top-100 following his victory.
I feel amazing. It was a hard week and a hard golf course. You can't be off your game. The people who played well are going to be on top," he said.
"I think, I played great and didn't miss too many shots. It was satisfying to master this course. Dave (his caddy) was saying that it was set up sometimes like the U.S. Open. You can hit great shots, but it can run into the rough or slope. So, it's particularly satisfying to win here."
Jyoti Randhawa and amateur Kshitij Naveed Kaul were the next best Indians in tied 32nd with totals of five-over 293. Kaul finished as the best amateur in the field.
Anirban Lahiri was tied 34th at six-over 294 total, while Khalin Joshi and Ajeetesh Sandhu took tied 40th place with scores of seven-over 295.
Karandeep Kochhar came in tied 48th at nine-over 297 while Udayan Mane was placed 52nd at 11-over 299.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
