Prithvi smashed an unbeaten 227 off just 152 balls to help Mumbai post a mammoth 457/4 against Puducherry.
Chasing a daunting target of 352, Railways were bowled out for 344.
This was Kerala's second victory on the trot in Elite Group C.
Kerala were 233/4 in 38.2 overs when rain ended play and were declared winners by VJD method against Odisha in Elite Group C.
Sreesanth thanked the fans for their support and assured that he will work hard in a video posted on Instagram.
Sachin Baby will lead Kerala in the forthcoming Vijaya Hazare Trophy.
Pacer S Sreesanth is also part of the 20-member team.
The BCCI opted for the Vijay Hazare Trophy as per the wishes of majority of the state units.
Mithun, who turned 30 on Friday, returned impressive figures of 5/34 to bowl out Tamil Nadu for 252 in 49.5 overs after opting to field.
The opener's third hundred of the competition helped Kerala end their campaign on a high.