McMahon won a bronze medal in the 400m freestyle at the 2010 European Junior Swimming Championships in Helsinki with a time of 4:15.92, an Irish junior record. In the 2011 Championships in Belgrade she won gold in the 400m freestyle with a time of 4:13.85 (more than 2 seconds faster than her time the previous year), another gold in the 50m breaststroke with a time of 32.00 (0.2 seconds faster the previous year's winner Lisa Fissneider), and a silver medal in the 200m freestyle with a time of 2:00.61, 0.11 seconds behind Russian Ksenia Yuskova.