Deepti Sharma, whose five-wicket haul helped India to win the 50-over World Cup final last year, took five wickets again for just 10 runs as Pakistan collapsed for 106, far short of the target of 171 in a battle of nerves in the Birmingham Group A clash.
"I varied my pace in every ball," said player-of-the-match Sharma.
"I always believe... whenever the time will come I'll step up for the team. That's how I play and bowl."
There was no handshake between the captains once again as India's Harmanpreet Kaur won the toss and chose to bat but her decision almost backfired when Shafali Verma and Jemimah Rodrigues fell in the first four overs.
But Smriti Mandhana and Kaur added 91 for the third wicket. Pakistan captain Fatima Sana (2-33) caught Mandhana (68) in the 14th over and got Kaur (36) caught soon after, arresting India's momentum.
Under pressure again Richa Ghosh (34 off 17 balls) dug India out of the rut, hitting Tasmia Rubab for three fours and a six to collect 23 from the 19th over and taking them to 6-170.
Pakistan openers Muneeba Ali and Gull Feroza scored 37 off Indian seamers in the first four overs, forcing Kaur to bring on spinner Sharma, dismissing Feroza and Ayesha Zafar in her first two overs.
Fellow spinner Shree Charani (3-21) sent back Saira Jabeen early, before opener Muneeba (41) was run out by Sharma's direct throw, leaving Pakistan at 4-75. Sana fell soon after for a duck, kicking off the collapse.
Sharma then wrapped up the match with three wickets in the 17th over.
Earlier at Edgbaston, Bangladesh avoided a scare in their opening match as they beat the Netherlands by six wickets.
Bangladesh, replying to the Netherlands' 8-139 were taken to the last over despite a great platform set by opening batter Juairiya Ferdous, who hit her second fifty since her T20 debut in January.
Both of her sixes flew over the mid-wicket rope and by the time she was out for 50 off 33 balls at 67-1 in the eighth over, Bangladesh were almost halfway home.
Dutch spinners Silver and Heather Siegers and Caroline de Lange (2-27) slowed down Bangladesh but an unbeaten partnership of 56 between Sharmin Akhter and Shorna Akter clinched Bangladesh's fourth win in seven T20 World Cups.
