While passing for 10,579 yards and 119 touchdowns during his prep career, Brohm led the Trinity Shamrocks to the 4-A state title in 2001, 2002, and 2003. He was the MVP of all three title games, the most exciting of which was a 59–56 victory over Louisville Male High School and 2002 Mr. Football Michael Bush. Brohm threw for 552 yards and 7 TDs in that game. Brian was featured on the cover of "Sports Illustrated" on November 18, 2002, while he was a junior in high school.