With overnight leader Chowrasia returning with five-over 76, Lahiri, who was seven strokes behind the Kolkata golfer after the third round, carded two-under 69 in regulation to go into the play-off after a roller-coaster day which saw fortunes fluctuating too often.
In the play-off, Lahiri, the top-ranked golfer in the field, didn't put a foot wrong as he fired a birdie in the 18th hole after Chowrasia failed to recover from a disappointing tee shot.
"It is incredible. Starting today, I didn't think I had a chance. It was one of the toughest days ever where I've played at Delhi Golf Club. This has been a childhood dream so it is very special to keep the Indian Open trophy at home. The way Siddikur and Chowrasia have been playing, it was hard to catch them," Lahiri said.
"Personally I wanted to play well and give myself a chance on the back nine. It was an important week for me. The leaders had a tough break and I feel lucky and fortunate.
"Honestly, it is hard to come to terms. Six months back, I was at Qualifying School so it feels like I've skipped a couple of steps to where I'm now," he added.
The tricky DGC bared its pangs on the final day as overnight leader Chowrasia stumbled on a bogey and double bogey to allow Siddikur Rahman, who was behind the Indian by two strokes, lead the course after four holes early in the day.