Casas was a member of the United States national baseball team at the 2020 Summer Olympics, which won the silver medal.

[2] He reclassified and graduated high school a year early to be eligible for the Major League Baseball (MLB) draft.

[4][5][6] The Boston Red Sox chose Casas in the first round, with the 26th overall selection, of the 2018 MLB draft.

[13] In mid-June, Casas was added to the top 100 prospects list of Baseball America, at number 98.

[20] During 2020, with no minor league season due to the COVID-19 pandemic shutdown, the Red Sox added Casas to their pool of reserve players on August 20, so he could participate in intra-squad workouts.

[30] Casas sustained a high ankle sprain in mid-May, causing him to miss over a month of playing time.

[31] The Red Sox promoted Casas to the major leagues on September 4,[32] and he made his MLB debut that day.

[33] In his debut, Casas recorded his first career hit, a single off of John King of the Texas Rangers.

Through 22 games, he had six home runs and 10 RBI, then was placed on the injured list on April 21 due to a rib injury.

[50] During the tournament, Casas hit home runs against South Korea, Japan, and the Dominican Republic.

[52] Casas' brother, Gavin, is currently a college baseball first baseman for the University of South Carolina Gamecocks.