Sony Norde opened the scoring with an extraordinary goal (50') and Nestor Gordillo (81') equalised later in the second half.
Sankar Lal Chakraborty made three changes to the side that lost 3-0 against Churchill Brothers. Shilton Paul, Dalraj Singh and Aser Dipanda were relegated to the bench as Sankar Roy, Omar Elhussieny and Lalchhawnkima got a starting berth.
On the other hand, Chennai City coach Akbar Nawas made two changes. Nauzet Santana was handed his debut and Pedro Manzi returned to the starting XI.
Mohun Bagan started the game on the ascendancy and could have drawn first blood in the 10th minute when Henry Kisekka set up Pintu Mahata inside the box. The winger took a touch and shot straight at Santana.
Bagan's continued to dominate and Omar pulled the strings from midfield. He sprayed passes to both the wings which allowed Norde and Pintu Mahata to use the space on the flanks. Norde was a constant menace on the left flank and kept Edwin Vanspaul on his toes throughout the match.
Santana looked unbeatable as he pulled off some magnificent saves to keep the scoreline unchanged when the players headed to the tunnel for the lemon break.
Bagan started on the same note in the second half as well and within five minutes of resumption, Norde put Bagan in front. Kisekka released a through ball for Norde and the Haitian cut inside from the left flank to set himself up for a crack at goal. He tried his luck from outside the box and the ball swerved into the far corner beyond the reach of Santana.
With 14 minutes left, Chakraborty brought on Shilton D'Silva in place of Omar and the momentum gradually shifted towards the away team. Within five minutes of the substitution, Chennai equalised when Roberto Eslava's hopeful ball found Nestor Gordillo in the penalty box. He showed excellent composure and his shot was potent enough to beat Roy under the sticks.
Chennai were a completely different side after the equaliser and had the woodwork deny them three points before the full-time whistle.
Both teams will get back to action on December 16 as Mohun Bagan will lock horns against arch-rivals East Bengal whereas Chennai will continue their journey on the road, against Minerva Punjab in Panchkula.