After he signed, he made his professional debut with the Lowell Spinners of the Low–A New York-Penn League,[5] and spent the whole season there, pitching to a 1–2 record and 2.28 ERA in 17 relief appearances.

In 2017, Poyner started the season with Salem, and was promoted to the Portland Sea Dogs of the Double-A Eastern League.

In 2018, the Red Sox invited Poyner to spring training as a non-roster player to compete for a major league role;[9] he was named to Boston's Opening Day roster.

[10] Poyner made his MLB debut on March 31, pitching 2⁄3 of an inning against the Tampa Bay Rays while allowing one hit, a home run by Carlos Gómez.

[11] Poyner recorded his first major league win on April 5, pitching two innings of scoreless relief, again against Tampa Bay.

Poyner was not included on Boston's postseason roster,[18] as the team went on to win the World Series over the Los Angeles Dodgers.

[25] Overall with the 2019 Red Sox, Poyner appeared in 13 games (one start), compiling an 0–1 record with 6.94 ERA and 11 strikeouts in 11+2⁄3 innings.