Muguruza defeated Anett Kontaveit 6-3, 7-5 in the final for her first title at the WTA's elite, season-ending championships.
She became the oldest champion at 28 since Serena Williams in 2014, and finishes the season at No. 3 in the rankings, her best performance since 2017, when she was No. 1.
Before the tournament, Muguruza took a 37-mile (60-kilometer) ride to Tequila, a small town outside of Guadalajara where the popular spirit is produced.
“I went to the town, did a tasting. I need to do this before because when I win ...” Muguruza said. “We’re (now) going to have some tequila, have fun, lose all the tension we’ve been having and holding. I think the whole team deserves it.”
Muguruza became the first Spaniard to win the WTA Finals. The first person she celebrated with was coach Conchita Martinez.
The new champion won back-to-back titles in Monterrey in northern Mexico in 2018 and 2019, and her past successes drove her to qualify for the WTA Finals.
"The whole year with my team I was saying to them, ‘It’s in Mexico, we have to make it, c’mon.' It was my biggest motivation," Muguruza said.
All week, she was backed by the locals at Estadio Akron, and reciprocated by wearing a Mexico soccer jersey for her last two matches.
“My manager, Oliver, he was like, 'You know what, for the first time Garbiñe, you’re really using the crowd, really getting that energy and using it on the court. You should do that more often,” Muguruza said.
Kontaveit will finish the year at No. 7. The Estonian qualified last for the WTA Finals by winning 29 of 32 matches, including four titles in 10 weeks.
She was broken only four times going into the final, but lost her serve five times to Muguruza.
The Spaniard made three breaks in the first set. Kontaveit played better in the second and got a break in the ninth game, but Muguruza broke her next two serves to win.
Earlier, Barbora Krejcikova and Katerina Siniakova defeated Hsieh Su-wei and Elise Mertens 6-3, 6-4 to win the doubles title undefeated.
“We needed to stay aggressive, and we did it,” said Siniakova, who finished the season No. 1 in the doubles rankings.