At 17 she became the youngest Irish medal-winner in a major event when she took silver in the 50 metre breaststroke at the 2012 European Aquatics Championships in Debrecen, where she also set a new Irish record. She competed for Ireland at the 2012 Summer Olympics, finishing 26th overall in the 100 metre breaststroke and 22nd overall in the 200 metre individual medley, where she came third in her heat. In the 2012 European Short Course Swimming Championships in Chartres she won bronze in the 50 m breaststroke behind Petra Chocová and Rikke Møller Pedersen. In 2013 McMahon took up a scholarship to study at Texas A&amp;M University. In her Freshman year she became the second-fastest A&amp;M Aggie ever in the 100 yard breaststroke when she finished fourth in the event at the SEC Championships with a time of 59.35 seconds.