Debuting with Adelaide as a 19-year-old, Mattner began his career slowly. After being elevated from Adelaide's rookie list at the start of the 2002 season, he played just seven games for the year. One of his best performances was a game against Geelong where he laid 13 tackles, a feature of his game that he had become noted for. He picked up 11 disposals in his first game, and then 13 disposals in a win over St Kilda. He averaged eight disposals a match over his seven games, and had an impressive average of just over three tackles a game.