Sitting on the outer in pursuing a Rugby League dream, McMahon was close to signing on with the Australian Army before being convinced by his brother, Brett, and manager to play at the 2011 Noosa Sevens tournament, kick starting his pursuit of a Gold jersey. An immediate success in the Sevens format, McMahon was a part of the Australian Sevens squad that won the bronze medal at the XX Commonwealth Games in Glasgow in 2014. McMahon also captained Australia at the 2014 U20s World Championships whilst juggling his debut season with the Melbourne Rebels.
- Australian Schoolboys 2011
- Australian U20s 2012 & 2014
- Australian Sevens Team 2012-2014
- Wallabies Test debut 2014