[3] Horn made his professional debut in 2021 with the Single-A Kannapolis Cannon Ballers, and also appeared for the High-A Winston-Salem Dash.
[5] He finished the year with the High-A South Bend Cubs, compiling a 4.98 ERA with 24 strikeouts across 7 games (4 starts).
[7] Horn split the 2023 season between the Double-A Tennessee Smokies and Triple-A Iowa Cubs.
[13] Horn was promoted to the major leagues for the first time on June 28,[14] and made his debut the following day, against the San Diego Padres.
[15] With the major-league Red Sox, Horn appeared in 18 games, all in relief, posting a 6.50 ERA while striking out 13 batters in 18 innings and recording one win and one loss.