[9][10] After signing with Oakland, Allen made his professional debut with the Rookie-level Arizona League Athletics, hitting .254 with one home run and 14 RBI over 35 games.

Allen spent the 2018 season with the Beloit Snappers of the Single–A Midwest League with whom he batted .239 with 34 RBI and 24 stolen bases over 121 games.

[12][13] He was placed on the injured list in late June after suffering a leg injury, and missed the remainder of the season.

[19] After returning from the Olympics in mid-August, Allen was promoted to the Las Vegas Aviators of the Triple-A West.

[26] On June 21, Allen hit his first major league home run, a two-run shot off of Seattle Mariners starter Marco Gonzales.

[30] On November 11, 2024, Allen was traded to the Atlanta Braves in exchange for minor league pitcher Jared Johnson.

[34] Following the Olympic competition, during which the United States won a silver medal,[35] Allen was named Best Defensive Player.