McMahon won bronze medals at the 2002 FESPIC Games in the 4 × 400 m relay, Men's Javelin F37 at the 2006 IPC Athletics World Championships and Men's 4 × 100 m T35-38 at the 2011 IPC Athletics World Championships. In 2006, McMahon received the 2006 Sports Achievement Award. At the 2008 Beijing Paralympics, he finished fourth in the Men's Javelin F37/38.