Arranda of the Tampa Bay Rays smacked an opposite-field shot into Houston's Daikin Park's Crawford Boxes to break the game open after Britain had hung in throughout the contest.
In front of a partisan crowd, Nacho Alvarez Jr. got the party started in the second inning, homering off of GB's starting pitcher Jack Anderson.
But, in truth, it was a difficult day for the 2023 semi-finalists who struggled at the plate as Great Britain's pitching got out of plenty of jams.
Harry Ford - Britain's catcher and breakout star of the WBC in 2023 - levelled the scores with a solo shot in the bottom of the sixth before Arranda settled the tie with a homer of his own, and his teammates scored a further four runs in the final inning of both teams' opening match of the WBC.
Mexico were firm favourites for the contest, with their line-up full of MLB talent including Randy Arozarena, Alejandro Kirk and speed merchant Jarred Durran.
Anderson was thrown into the deep end starting up for the Brits, but he held his own, conceding just one run as he threw three innings of two-hit ball.
His opposite number, Javier Assad of the Chicago Cubs, went 3.2 innings with two hits and three strikeouts. Both bullpens had their struggles, but the game was still tied as it entered the top of the seventh.
Up stepped Arranda. The burly first baseman cashed in walks from Duran and Arozarena to set Houston alight, with the Mexican fans going wild as the ball sailed over the fence.
His reaction showed just what it meant to him and his teammates. Despite the mismatch in perceived quality with a 346-foot shot, every win is vital at the WBC, and Arranda slapped the dugout with gusto after all but sealing the win.
They would tack on another two runs in the ninth, Alek Thomas squeezing a single up the middle to score Nick Gonzalez from third base and Jared Serna from second. Thomas would score himself soon after, running around the bases to make it 7-1 after a double down the right-field line from Joey Ortiz to end the contest.
Arozarena would then also get a hit to cash in another before Arranda would strike out to end the carnage.
Star Seattle Mariners closer Andreas Munoz then came into the game for Mexico, and GB were able to score a consolation run off a double from Trayce Thompson.
For Great Britain's biggest name, Jazz Chisholm Jr of the New York Yankees, it was a disappointing day. He went 0-4 in the game, and he will hope for better in their upcoming clashes.
It gets no easier for the British side on Saturday night/Sunday morning, as they take on tournament favourites the USA in Houston from 02:00 CET, where they will face reigning Cy Young holder, Tarik Skubal.
Mexico have a rare day off on Saturday in this congested tournament, with their next game in Houston coming against Brazil on March 9th at 01:00 CET.
Follow all the action from the World Baseball Classic with Flashscore.