A goal each from Keane Lewis and Sehnaj Singh in either half confirmed Mohun Bagan a crucial win in the away match in this two-legged fixture.
It took Mohun Bagan just 13 minutes to score the opening goal. Keane Lewis headed in comfortably the opener from Pronay Halder's cross.
Sanjay Sen did not field a half fit Sony Norde in this match. Defender Eduardo too was dropped from the squad due to injury.
The gaffer made five changes in the starting XI. Keane Lewis filled in place of Norde. Shilton Paul got his first start this season under the bar.
Mohun Bagan dominated the proceedings in the first half but the home side pulled one back at the half-hour mark Afeez Olofin scored the equaliser with a great strike. Shilton remained completely clueless.
In the second half, once again the visitors took control of the game and played several passes on the ground but could open the goal mouth.
After a series of attacks, Bagan finally scored the winner thanks to a brilliant strike from Sehnaj Singh in the 70th minute.
Sanjay Sen inducted Balwant Singh into the game in the latter stages substituting Darryl Duffy.
The Indian outfit played quite average but managed to pick up a vital win against a much weaker Colombo FC side.
The Sri Lankan side next travel to Kolkata on February 7 to play the second leg.