McMahon was elevated off the rookie list permanently at the start of the 2007 AFL season and made his his senior debut against Carlton Football Club in Round Eight, collecting 13 disposals. McMahon was soon dropped back to the VFL and was included as an emergency for North Melbourne several times before he returned to the seniors for the Round 13 clash against the Western Bulldogs. In that match he kicked two goals and was rated "one of the Roos' rising midfielders" by the media. He played the next week against Richmond Football Club and again kicked two goals, matching his previous week's performance. McMahon was rated by then North Melbourne coach Dean Laidley as "the best young player we got for nothing". For his performance in that game, he won the Round 14 AFL Rising Star nomination.