Due to the timing of his Grand Prix events, Mroz had a bye to the 2009 U.S. Championships. At Nationals, Mroz placed 4th in the short program after being credited with landing a 3A as his required element axel jump and a 3Lz-3T as his combination jump. He placed 2nd in the free skate, in which he was credited with landing a 4T and eight triples, including a 3Lz-3T combination. He won the silver medal overall, placing 12.19 points behind champion and training-mate Jeremy Abbott and 0.60 points ahead of Evan Lysacek.