Iyer combined with opener Bhavin Thakkar (30) to share an 88-run stand before Aditya Tare (68) and Abhishek Nayar (43) played a handy knock to give their team a solid start to the semifinal.
Siddhesh Lad was batting on 57 at stumps in the company of Shardul Thakur in the game played at DRIEMS Ground here.
The star bowler for Madhya Pradesh turned out to be pacer Chandrakant Sakure, who had made his First-Class debut in the quarterfinal against Bengal.
While Mumbai are 40-time winners of the prestigious domestic competition, Madhya Pradesh have entered the final only for the second time.
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Iyer's 104-ball effort came after Mumbai lost Akhil Herwadkar early, having put in to bat by the opposition.
He scored runs at a brisk pace and ended with 13 fours and a six.
Tare took over from Iyer and played a patient knock off 117 balls, highlighted with 10 fours.
Brief scores: Mumbai 327/7 in 89 overs (Shreyas Iyer 90, Aditya Tare 68; Chandrakant Sakure 4/102, Ishwar Pandey 2/34).