Looking to return to winning ways, Untied SC started the match on a positive note when Ranty Martins gave the purple brigade the lead in the 29th minute.
The home side led 1-0 at the half-time but only to see Mumbai FC equalise through Evams Quao via a header in the 52nd minute in an intense second-half.
Substitute Abhishek Yadav, who replaced Gabriel in the 78th minute, made his presence felt when his 82nd minute goal took the side go past United SC.
United did not give up and they equalised in the 90+2nd minute through Shankar Oraon.
But more action was in store when Mumbai's Ghanian recruit Yakubu made it 3-2 for Mumbai FC in the 90+3rd minute to hand them a brilliant win.
Mumbai FC shot themselves to top-half with 13 points, while United SC remained in the bottom half with six points from nine matches.