He has previously played in Major League Baseball (MLB) for the Texas Rangers, Atlanta Braves, and Kansas City Royals.

[2][3] Hearn spent the 2015 season with both the rookie GCL Nationals and the Low–A Auburn Doubledays, posting a 1–5 record and a 3.56 earned run average (ERA) in 12 games (11 starts).

On July 30, 2016, the Nationals traded Hearn, along with Felipe Vázquez, to the Pittsburgh Pirates for Mark Melancon.

Hearn played the entire 2017 season with the High–A Bradenton Marauders, accumulating a 4–6 record and a 4.12 ERA in 19 games (18 starts).

[5] Hearn was traded to the Texas Rangers, along with Sherten Apostel, in exchange for Keone Kela on July 30, 2018.

[10] Hearn was promoted to the Rangers on April 25, 2019, to make his major league debut that night against the Seattle Mariners.

He bounced between the rotation and bullpen that season, as well as being demoted to the Round Rock Express of the Triple-A Pacific Coast League for three games.

[18] On January 13, 2023, Hearn agreed to a one-year, $1.4625 million contract with the Rangers, avoiding salary arbitration.

[26] On July 30, 2023, the Braves traded Hearn to the Kansas City Royals in exchange for Nicky Lopez.

[27][28] On August 8, Hearn was promoted to the major leagues, and he made his Royals debut two days later, pitching an inning in relief, allowing one hit and no runs.