[10] The Toronto Blue Jays selected Horwitz in the 24th round of the 2019 Major League Baseball draft.
[5] Horwitz spent his first professional season in 2019 with the Rookie League Bluefield Blue Jays and Low–A Vancouver Canadians.
[14] Horwitz did not play in 2020 due to the Minor League Baseball season being cancelled because of the COVID-19 pandemic.
Horwitz returned in 2021 to play for the now High–A Vancouver Canadians and for the Double–A New Hampshire Fisher Cats.
[7] With Vancouver, he led the High-A West with 28 doubles and 70 walks (to only 66 strikeouts), and had a .401 on-base percentage (9th) with 65 runs (6th), 62 RBIs (5th), and 5 sacrifice flies (3rd).
[20] Horwitz started 2022 with New Hampshire, and was named Eastern League Player of the Week on June 13.
[25] On June 16, 2023, Horwitz was promoted to the major leagues for the first time after outfielder Nathan Lukes was optioned to Triple–A.
[30] He hit his first major-league home run (a solo shot) in a 7-5 win against the Colorado Rockies on September 3, 2023.
[32] Spencer said: “Jay was our public relations guy for Team Israel during the WBC.... And it just kind of blew up.
"[32] In January 2024, MLB Pipeline ranked Horwitz #9 on its Top 10 1B Prospects list.
[34][35][36][37] Horwitz was called up to the Blue Jays on June 6, 2024, after the club designated Cavan Biggio for assignment.
[40] On the same day, Horwitz was then traded to the Pittsburgh Pirates in exchange for Luis Ortiz, Michael Kennedy, and Josh Hartle.