Brethowr attended Saint Thomas Aquinas High School in Kansas. He had an impressive high school campaign which earned him a ranking as the 10th best prospect and the 2nd best outfielder in the state of Kansas. He chose to join Arizona State following his high school career.
As a freshman with ASU, Brethowr played just 16 games with 7 starts, slashing .290/.395/.516 (average, on base %, slugging). He drove in 6 RBI with 2 home runs and a double in 31 at bats. He entered the transfer portal following this freshman season.
Brethowr chose to transfer to UC Santa Barbara for his sophomore year. He made an immediate impact as a transfer starting 45 games for UCSB. He slashed .311/.408/.571 as a sophomore, driving in 40 RBI on 11 home runs and 13 doubles. He also stole 8 bases. He earned an All-Big West Selection for this sophomore showing.
This year is Brethowr's junior season. He is expected to continue his success at the plate and climb draft boards ahead of this season's draft.