[2] He signed for $1.3 million, forgoing his commitment to play college baseball at the University of Central Florida.

In 2016, Mountcastle spent the season with the Delmarva Shorebirds where he posted a .281 average with ten home runs and 51 RBIs along with a .745 OPS in 115 games.

[5] In 2017, Mountcastle spent time between the Frederick Keys and the Bowie Baysox, batting a combined .287 along with a career high 18 home runs and 62 RBIs in 127 games between both teams.

[15] On August 22, Mountcastle had his first major league hit against the Boston Red Sox in the bottom of the ninth inning.

[18] Mountcastle returned to Baltimore for the 2021 season as their starting first baseman and also spent time in left field and as their designated hitter.

On June 7, 2021, Mountcastle was named American League Player of the Week, after hitting safely in all six games he played in, and going 11-for-24 (.458) with four home runs, three doubles, ten RBIs and a 1.563 OPS.