In 2015, Smith was called up to the majors for the first time on April 13,[2] but did not make his MLB debut until June 23, allowing four runs in three innings of a no decision against the Pittsburgh Pirates.

[5] In 2017, Smith began the season with the Triple-A Nashville Sounds, until the Athletics purchased his contract on May 3, adding him to their active roster.

[7] He made four appearances with the Tacoma Rainiers of the Pacific Coast League, recording a 6.10 ERA,[4] and was released on April 21, 2018.

[8] He joined the Triple-A Pawtucket Red Sox in June, appearing in 18 games (10 starts) with a 5–6 record and 4.14 ERA.

[4] He was added to Boston's active roster on April 26 to make a spot-start against the Tampa Bay Rays;[9] however, the game was rained out.

[15] After spending all of July with Pawtucket, Smith was recalled to Boston on August 2, when Heath Hembree was placed on the injured list.

[17] Overall with the 2019 Red Sox, Smith appeared in 18 games (two starts), compiling an 0–3 record with one save, along with a 5.81 ERA and 29 strikeouts in 31 innings.

Smith played in 9 games between the Florida Complex League Marlins and the Triple-A Jacksonville Jumbo Shrimp, registering a 2–0 record and 2.60 ERA.