Brennan is from De Beque, Colorado and played high school basketball at Wasatch Academy (Utah). At Wasatch, he become an internet sensation after a dunk of his went viral on the popular social media account BallIsLife. He spent the 2021-22 season playing for Northwest Florida State College (JUCO), where he led his team to a NJCAA national championship averaging 11.6 points, 3.6 rebounds, and 1.2 assists in 33 games. Currently, he is a sophomore on the Oregon Men's Basketball team. Through his 6 games played (4 starts) for the Ducks this season, Brennan is averaging 8.2 points, 4.0 rebounds, and 1.3 assists in 32.3 minutes per game.