With this victory, India Women have taken a 1-0 lead in the three-match series over Sri Lanka.
Put in to bat, India scored 245 for six in their allotted 50 overs and then bundled out Sri Lanka for a meagre 138 in 45.2 overs.
Besides the two half-centurions, skipper Mithali Raj also chipped in with a crucial 49.
Opener Mandhana anchored the first half of the innings with a solid 55 that came off 81 deliveries and was studded with eight fours.
After Mandhana's dismissal, Harmanpreet took charge with a 61-ball 50, while Mithali's 49 off 74 deliveries also came in handy for the team.
Chasing 246, Sri Lanka lost Chamari Jayangani (1) early, but the other opener in Prasadani Weerakkody played a patient innings of 113-ball 69 to steady the boat.
At one stage, Sri Lanka were decently placed at 122 for three before Indian bowlers led by Poonam cut through the middle and lower order.
Sri Lanka lost their last seven wickets for just 16 runs.
India Women: 245 for six in 50 overs (Smriti Mandhana 55, Harmanpreet Kaur 50; Shashikala Siriwardene 2/31).
Sri Lanka Women: 138 all out in 45.2 overs (Prasadani Weerakkody 69; Poonam 4/22).