Young goalkeeper Debjit Majumder made all the difference with a standout performance with as many as half a dozen stunning saves to earn a second successive Man-of-the-Match award in front of a packed 50,000-odd crowd.
It was Balwant Singh who put his name on the board with his 47th minute effort from a Katsumi Yusa cross to give the green and maroon brigade their first I-League derby win since November 20, 2011 when they had won by an identical margin.
It was an agonising defeat for the other city heavyweights and last season's runner-up East Bengal who came close on so many occasions but a goal simply eluded them to suffer their second defeat from nine matches.
East Bengal (13 points) will have a herculean task now to replicate last season's show.
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Majumder, who was also adjudged MoM in their 1-0 win against Dempo in the last outing here, credited the team for their win.
"The win was very important to stay on top. I'm not the only reason behind the win. Everybody gave their 100 per cent so we won the match," a humble Majumder said.
Yusa picked up the ball from the right side of the box and dribbled past Robert Lalthlamuana who was completely at sea.
The Japanese's strike however was deined by Subhasish Roy Chowdhury as the ball ricocheted to Balwant who scored with ease with the ball going past the East Bengal captain Harmanjyot Khabra's legs.
Having scored against the run of play, Mohun Bagan were all over East Bengal who suddenly paled in comparison.