After some half-hearted attempts, Mohun Bagan's 'lucky charm' Balwant gave them the lead with his 28th minute header against the only Goan team in the league.
Making a comeback after nine months, the fit-again Balwant found himself unmarked as he capitalised on Pritam Kotal's precise cross from the right flank before heading the ball into the net.
Down to 10 men in the second-half, the threat of conceding remained till the last minute with some swift freekicks, but Mohun Bagan returned lucky to earn full three points in their opening match.
Subhasish was warned for the first time in the 55th minute after he had pushed down the visitors captain Keenan Almeida near the touchline.
Churchill had a brilliant opportunity to equalise in the 70th minute when Seriton went past Prabir Das and had only Debjit Majumder to beat under the bar, but his shot went off the mark, much to the home team's respite.
Four minutes left for the regulation time, Agnelo's powerful shot missed the target after Churchill earned a freekick following Pronay Halder's foul.
The 2012-13 champions Churchill Brothers, who were making a comeback into the top division after four seasons, would certainly return disappointed as they came close to scoring on many occasions.
Churchill also were dealt a blow when captain Keenan Almeida suffered a bruise after a challenge with Balwant and had to be stretchered off in the 17th minute, but he returned with a bandaged head to pep up his troops.
The star Haitian sent in a fine cross to the far post where Balwant timed his header perfectly to find the net for their much-awaited equaliser.
With four minutes added, Mohun Bagan had a last shot at goal with Jeje Lalpekhlua, but his acrobatic effort flew over.
Earlier, in the 75th minute, Balwant was unlucky as his header hit the post.
Mohun Bagan also suffered an injury blow when their main goalkeeper Debjit was stretchered off the ground after being hit while getting hold of the ball in a battle with Fernandes as Shilton Paul was brought in.
Mumbai, on the other hand, remained in the relegation zone with 10 points from 13 matches.
