Mark's father became a fan of the New York Mets, whose rooting interest and being taught how to play the game, had passed onto him as well.

[10][11] Vientos signed with the Mets for $1.5 million, forgoing his commitment to play college baseball at the University of Miami.

[14] In 2019, he played with the Columbia Fireflies of the Single–A South Atlantic League, hitting .255/.300/.411 with 12 home runs, 62 RBI, and 27 doubles over 111 games.

[19] After slashing .281/.346/.580 with 22 home runs and 59 RBIs over 72 games, he was promoted to the Syracuse Mets of the Triple-A East in early September.

[30] On September 24, Vientos hit his first major league home run, off Oakland Athletics starter Ken Waldichuk.

Vientos was again optioned to Triple–A Syracuse to begin the 2024 season,[34] but was later called up for the remainder of a series against the St. Louis Cardinals after Starling Marte was transferred to the bereavement list.

[39] In his postseason debut, Vientos hit a go-ahead 2-RBI single in Game 1 of the 2024 National League Wild Card Series.

[41] In Game 2 of the 2024 National League Championship Series against the Los Angeles Dodgers, Vientos hit his first career grand slam in the 2nd inning giving the Mets a 7–3 win.

[45] Vientos has appeared at the youth level for the United States and captured a silver medal at the 2014 15U Baseball World Cup.

[4] Prior to the 2023 World Baseball Classic, it was initially reported that Vientos would play for Nicaragua but he ultimately did not appear on the team's roster opting to instead participate in practices with his Mets teammates throughout the spring.