The sixth-ranked Garcia became the second Frenchwoman to win the season-ending event after Amelie Mauresmo in 2005, also the last time the WTA Finals were held in the US.
There was only one break point in a meeting of the tour (Garcia) and tournament (Sabalenka) leaders in aces coming into Monday night's match.
Garcia converted that in the first game of the second set while handing No.7 Sabalenka just her third loss in 12 career hard-court finals on the temporary indoor court at Dickies Arena in Fort Worth.
The WTA Finals were moved to Texas from China over concerns about the safety of Peng Shuai, a grand slam doubles champion who accused a former government official there of sexual assault. Coronavirus restrictions also played a part in the decision.
Sabalenka, trying to become the first player from Belarus to win the event, reached the title match by beating top-ranked Iga Swiatek.
Garcia matched Sabalenka's power from the start in her fourth victory of the season, second only to Swiatek's eight. The 29-year-old is the oldest WTA Finals winner since Serena Williams at 33 in 2014.
Garcia won six consecutive points in the tiebreaker, capped by her 10th ace of the first set for a 6-2 lead before Sabalenka's third double fault ended the set.
Garcia opened the second set with two break points, but only needed one. Her forehand winner on Sabalenka's serve put her ahead for good.
Earlier in the day, Veronika Kudermetova and Elise Mertens beat defending champions Barbora Krejcikova and Katerina Siniakova 6-2 4-6 11-9 to win the doubles title.Â
Krejcikova and Siniakova were trying to finish off a dominant season that included titles at all three of the grand slam events in which they competed. It was just the fourth loss of the season for the Czech duo, who were trying to become the seventh doubles team to defend a WTA Finals championship.
Russia's Kudermetova and Mertens of Belgium avenged a loss to Krejcikova and Siniakova in the Australian Open semi-finals.
Mertens is a three-time major winner in doubles, including the 2019 US Open and 2021 Australian Open with Sabalenka.