A first-half goal from Diego Carlos (44') was all that Jorge Costa's men needed to notch an important win at home while NorthEast United went a fourth successive game without scoring.
The win sees Mumbai climb to fourth on the table, two points clear of fifth-placed Odisha FC, while NorthEast, winless in nine games, are second from bottom.
After a scrappy start to the game, the teams showed intent in the opening exchanges but could not fashion any clear cut chances. Diego Carlos looked threatening for Mumbai City while Simon Lundevall worked hard in midfield for NorthEast United.
The first real opening of the game came in the 28th minute when a cross from Carlos fell for Rowllin Borges inside the box. His host took a deflection and sat up for Modou Sougou who saw his effort come back off the upright.
The game continued to be even with both teams looking threatening in possession. Just when it seemed like the teams would go into the break level, Carlos took matters into his own hand. A cross from Mohammed Larbi was headed out straight to Carlos inside the penalty box. The Brazilian shifted the ball onto his left foot and found the bottom corner with a fine effort.
NorthEast coach Robert Jarni, in an effort to turn the situation around, brought on Martin Chaves for defensive midfielder Jose Leudo at the break.
However, Mumbai continued to threaten at the start of the second half. Sougou found a lot of joy down the right-wing and it required a superb tackle from Provat Lakra in the 58th minute to stop him in his tracks.
Four minutes later, however, Sougou found a lot of space down the wing on a counter before flashing a shot agonisingly wide, just beyond the reach of Amine Chermiti as well, who had slid in at the far post.
A few minutes later, Lundevall worked some space to shoot inside the Mumbai box but his feeble effort was easily held by Mumbai goalkeeper Amrinder Singh.
NorthEast did push for an equaliser, with Lalengmawia going close with a header. But Mumbai had no problems in seeing the game out, with NorthEast's problems compounded by the late sending off of Reagan Singh, who saw two yellow cards in quick succession.