Joshimar, who was played as a lone striker after coach Abdul Masood Bola relegated Tolgay Ozbey to the bench, struck the solitary goal from the spot.
But Penn Orji was instrumental in getting the penalty as his lovely cross found Ajay Singh before hitting Ritesh Perambra's arm for the referee to point to the dreaded spot.
The Brazilian could have easily doubled the tally in the 53rd minute but he failed to find the target.
The visiting side missed a sitter in the second minute when Sandeep Sangha almost scored an own goal trying to clear Nicholas Rodriques freekick but an alert Ashok averted a mishap.
Mohammedan Sporting now have 10 points from eight matches, while the Mumbai outfit remained on seven from nine outings.