He has previously played in MLB for the Cleveland Indians, Seattle Mariners, Baltimore Orioles, Miami Marlins, Tampa Bay Rays, St. Louis Cardinals, and Chicago Cubs.

Armstrong attended West Craven High School in Vanceboro, North Carolina.

[1] He was a member of the varsity baseball team for his entire high school career and participated in cross country for two seasons.

He missed the 2009 season after having shoulder surgery, and finished his collegiate career with a 4-3 win–loss record and a 4.45 earned run average (ERA).

[9] Armstrong made his major league debut the same night, pitching a scoreless ninth inning with two strikeouts.

On December 13, 2017, the Indians traded Armstrong to the Seattle Mariners for international bonus pool money.

[12] Armstrong was designated for assignment on April 26 after struggling to a 14.73 ERA in 4 appearances to begin the year.

[21] On April 7, the Marlins selected Armstrong's contract, adding him to their opening day roster.

On May 10, 2022, Armstrong signed a minor league deal with the Tampa Bay Rays and was assigned to the Triple–A Durham Bulls.

[23] Armstrong made 43 appearances for Tampa Bay in 2022, posting a 2–3 record and 3.60 ERA with 61 strikeouts and two saves in 55 innings pitched.

[26] On July 30, 2024, the Rays traded Armstrong to the St. Louis Cardinals in exchange for Dylan Carlson.