Pathirana, who is called 'Baby Malinga' for his slinging action similar to former Sri Lankan pace ace Lasith Malinga, returned figures of 3-15 to limit five-time winners Mumbai to 139-8 in Chennai.
New Zealand opener Devon Conway made 44 as M.S. Dhoni's Chennai -- four-time IPL champions -- romped home with 14 balls to spare.
Dhoni praised Pathirana, saying, "People who don't have any clean action, batsmen have a difficult time picking it. But his consistency, pace makes him special."
Dhoni advised the youngster to stay away from Test cricket and concentrate on playing "only ICC tournaments with ODIs".
The left-handed Conway put on a strong opening stand of 46 with Ruturaj Gaikwad and moved second in the batting chart with 458 runs behind Royal Challengers Bangalore skipper Faf du Plessis, who has 466.
The 41-year-old Dhoni hit the winning run at Chennai's MA Chidambaram Stadium as fans, who are coming in numbers to cheer the superstar cricketer in what it is expected to be his last IPL season as player, went wild.
Shivam Dube hit an unbeaten 26 off 18 balls as Chennai jumped a spot to second behind table-toppers Gujarat Titans in the 10-team table.
Chennai's bowlers set up victory after they rattled the Mumbai top-order including the key wickets of Cameron Green, out for six after opening the batting, and skipper Rohit Sharma, dismissed for a second successive duck after coming at number three.
The opening pace duo of Deepak Chahar and Tushar Deshpande reduced Mumbai to 14-3 inside three overs before Nehal Wadhera (64) and Suryakumar Yadav (26) attempted to rebuild in their stand of 55.
Left-arm spinner Ravindra Jadeja sent back Suryakumar and the 20-year-old Pathirana, who has played one T20 for Sri Lanka, bowled Wadhera with a yorker - his main weapon - to take the steam out of the Mumbai batting.
Pathirana with a unique celebration of his hands in front of his chest and eyes in meditation said, "I'm a die-hard fan of Cristiano Ronaldo and have modelled my celebration after him."
Wickets kept tumbling as the dangerous Tim David fell for two off Deshpande and Pathirana got South African batsman Tristan Stubbs for 20.
