Opting to bat, Maharashtra made 271 for seven and then restricted Mumbai on 196 for nine at the Madhavrao Scindia Cricket ground.
Maharashtra, who were reduced to 136 for six, managed to post a competitive total because of half centuries made by Naik and Mundhe.
Naik, (19), playing in his third list A match, made 78 runs off 98 balls with six fours and two sixes, while Mundhe made a career-best unbeaten 72, that came off 62 balls with six fours and three sixes. The duo added 109 runs for the seventh wicket in 19 overs.
For Mumbai, Saurabh Netrawalkar took two wickets conceding 70 runs, while Abhishek Nayar had 2 for 23.
Chasing 272, Mumbai never recovered from the early damage done by Samad Fallah, who dismissed Shoeb Sheikh (22), Sushant Marathe (2) and Iqbal Abdulla (0).
Lower-order batsmen Siddharth Chitnis and Dhaval Kulkarni made 38 and 39 runs, respectively.
For Maharashtra, Fallah was the pick of the bowlers taking three wickets and giving away 27 runs, while Ankit Bawne took 2 wickets conceding 28 runs.
With this loss, Mumbai have failed to qualify for the knockout stage losing three matches in the tournament.
Maharashtra 271 for 7 off 50 overs (Nikhil Naik 78, Srikant Mundhe 72, Saurabh Netrawalkar 2-70)
Mumbai 196 for 9 off 50 overs (Dhaval Kulkarni not out 39, Siddhart Chitnis 38, Samad Fallah 3-27).