[1][2] Undrafted out of high school in the 2014 MLB draft, he enrolled at Central Connecticut State University, where he played college baseball.

[6] He returned from the injury that summer and played for the Keene Swamp Bats where he hit .264 with five home runs and 19 RBI in 42 games.

[9] He signed for $5,000[10] and made his professional debut that year for the rookie–level Arizona League Mariners, batting .331/.430/.634 with eight home runs and 38 RBI in 44 games.

He began 2018 with the Single–A Clinton LumberKings,[11] with whom he was named to the Midwest League All-Star Game[12] where he participated in the Home Run Derby.

[13] On July 30, 2018, Costello (along with Chase De Jong) was traded to the Minnesota Twins in exchange for Zach Duke.