Yusa Katsumi had earlier converted from the spot to put ONGC in the lead in the 18th minute. The match marked the end of ONGC's home matches here.
Courtesy today's result, both teams moved to 31 points from 25 matches.
ONGC's first chance came as early as the fifth minute. Kailash Patil's deft turn caught rival defender Kamaljeet Kumar on the wrong foot and the former was brought down. But off the resultant free-kick, Yusa Katsumi's try sailed inches over the bar.
Mumbai FC came back strongly and forced a goal-line clearance in the 12th minute -- Justin Stephen's header off a Nicolas Rodrigues corner was cleared off the line by Sachin Gawas.
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ONGC surged ahead in the 18th minute. A David cross from the right was handled inside the box by Kamaljeet and Yusa gleefully converted from the spot to make it 1-0.
Mumbai FC's best chance in the first-half came in the 31st minute. Peter Costa set up Evans Quao inside the box but his prompt shot was palmed away by ONGC goalkeeper Rehenesh.