It was their first away match and Sanjoy Sen's boys did well to come back from a goal behind after Brazilian Marcos Tank's 52nd minute goal put the local team in lead.
However the joy was shortlived with India international Jeje Lalpekhlua tapped home from close range in the 56th minute.
Bagan now have 12 points from four games and along with Bengaluru FC are firm contenders for the I-League title as it has been the case for the last few seasons.
Mohun Bagan, on the other hand, made just the one change with with Bikramjit Singh starting the match.
While Karanjit pulled off two good saves off Darryl Duffy's attempts, he was splendid as he dived to his right in the 38th minute to save a spot-kick taken by Norde.
The penalty was awarded after stopper Joseph Clemente brought down Norde in the box. But Karanjit proved to be equal to his task.
In what was a blow for Chennai, captain D Ravanan was injured late in the first half, with former Mohun Bagan mideo Denson Devadas replacing him. The first half finished goalless.
But Mohun Bagan equalised within four minutes when Norde did well to trick Clemente and found Duffy inside the box. The Scotsman's shot was blocked but Jeje was on hand to volley home from close range.
The winner in the 77th minute was as much as Sony Norde's as it was Jeje's, who slipped in a sublime through pass with the Haitian sprinted past his marker and beat a surging Karanjit with a deft instep placement.
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