Kulkarni's pacemates Abhishek Nayar and Sagar Tivedi took three wickets apiece to help Mumbai bundle Assam -- chasing 199 for a win -- out for 102 runs in 35 overs at Railways Recreation Club Ground.
Batting first, Mumbai began strongly reaching 60 for 1 before suffering a middle-order slump to fall to 118 for 7 and then ended their innings at 198 all out in 49.54 overs.
The first half of Mumbai's innings was held together by Shreyas Iyer's 65-ball 49. Once Iyer departed -- one of the six wickets Mumbai lost for 58 runs -- Suryakant Yadav's 46-run partnership with Dhawal Kulkarni (21 not out) lifted the side past 150.
Kulkarni then dismissed Assam openers -- Pallavkumar Das and Sibsankar Roy -- within 11 overs after which Nayar and Trivedi ran through the middle-order.
Amit Verma's 33 and J Syed Mohammad's 25 were the only scores above 20 as Assam crumbled to 83 for 8 by the 29th over.