He has previously played in Major League Baseball (MLB) for the Toronto Blue Jays, Chicago Cubs, and New York Mets.

[4][5][6] The New York Mets selected Kay in the first round of the 2016 Major League Baseball Draft (31st overall) using the compensatory pick they received after the Washington Nationals signed Daniel Murphy.

[7] Kay did not appear in any games in the Mets' organization after being drafted and then underwent Tommy John surgery on October 4, 2016, putting him out for the entire 2017 season.

[10] On July 28, 2019, the Mets traded Kay and Simeon Woods Richardson to the Blue Jays for Marcus Stroman and cash considerations.

With the 2020 Toronto Blue Jays, Kay appeared in 13 games, compiling a 2-0 record with 5.14 ERA and 22 strikeouts in 21 innings pitched.

[13] Kay dealt with an unspecified injury for most of 2022 and only made one appearance for the big-league club, in which he allowed one run in two innings of work.

[20] He spent most of his stint with the Triple–A Syracuse Mets; in four appearances for New York, he struggled to a 7.36 ERA with three strikeouts in 3+2⁄3 innings of work.