Brennan joined Hawaii in 2005 and quickly earned the starting quarterback job. He started 10 of 12 games, the only games he did not start being against USC and San Diego State. He either tied or broke 11 school offensive records in what was a successful first season with the Warriors. He led the country in total offense yards (4,455) and touchdowns thrown (35). His 4,301 yards passing is the eighth-most in Western Athletic Conference (WAC) history. Against New Mexico State, he recorded career-high numbers in passing yards (515), touchdowns (7), and pass completions (38). He also had nine 300+ yard performances on the season, including four 400+ yard games and a 515-yard performance.