Ranked 20 in the world, Garcia battled her way through two tight sets against Barty before breezing the third to claim the biggest title of her career.
It was the five-time Grand Slam winner`s first appearance since she was beaten by Sevastova at the US Open in early September and gets her stop-start return from a doping ban back on track.
The build up to the match had been dominated by the international suspension of all rounder Ben Stokes and opener Alex Hales after the pair were involved in an altercation outside a Bristol nightclub but the off-pitch events failed to distract England from trouncing West Indies.
The one-drop batsman was unbeaten on 74 but saw Babar Azam fell for 28 in the last over of the day as Pakistan finished on 266-4 on a Sheikh Zayed Stadium pitch which has started to assist spinners.
The Brazilian curled in a brilliant 30-yard free-kick after just five minutes at the Parc des Princes before sliding in Edinson Cavani to double the lead seven minutes later.