Arenado collected an RBI single in the first inning to cancel out an early Yankees run, and after the visiting side jumped ahead 4-1 in the second frame thanks in large part to a two-RBI base hit from AL MVP favourite Aaron Judge, it all began to fall apart.
Dylan Carlson's RBI double started the rally and cut the margin to 4-2, a sacrifice-fly from NL MVP favourite Paul Goldschmidt made it 4-3, and then Arenado connected on a three-run home run to lead 6-4 after two innings.
Montas would be pulled to begin the fourth frame, finishing with figures of six earned runs from five hits and three walks in his 64 pitches.
To the Yankees' credit, they refused to lay down, with Judge driving in another two runs with a double in the fifth inning to tie the game at 6-6, but the Cardinals continued to answer right back, adding three more runs of their own in the bottom of the fifth.
The Yankees again cut the lead to 9-8, but Paul DeJong put the game to bed in the eighth inning with a three-run homer to grab a winning break.
Overall, the teams combined for 21 runs from 27 hits, with Arenado going three-for-five at the plate with four RBIs, and Judge finishing two-for-five with four RBIs. Judge now leads the majors in RBIs with 97, two more than New York Mets slugger Pete Alonso.
DeGrom makes his case as the game's best pitcher
There were concerns that perhaps injuries would strip New York Mets ace Jacob deGrom of his status as the sport's most elite pitcher, but he continued to dispel that myth on Sunday in his side's 5-2 win against the Atlanta Braves.
DeGrom, in his second start of the season, struck out 12 of the 19 batters he faced, going on to finish with two earned runs from one hit and one walk in five-and-two-thirds innings.
Through five innings, deGrom had a perfect game with 10 strikeouts, but was pulled in the sixth after his first walk of the game was followed by a home run from Dansby Swanson to cut the Mets' lead to 5-2, with both bullpens keeping things scoreless the rest of the way.
Rays pull off improbable late rally
The Rays as a team conceded only three hits and no walks as six pitchers combined beautifully, while the Tigers relied on Matt Manning to get them through most of the game, pitching seven scoreless frames for seven baserunners and seven strikeouts.
In the ninth inning, after two outs, the Rays rattled off consecutive at-bats resulting in a double, three walks, a single, a double and another single as seven straight batters reached base.