|Competitor for Ireland|
|European Championships (LC)|
|Silver||2012 Debrecen||50 m breaststroke|
|European Championships (SC)|
|Bronze||2012 Chartres||50 m breaststroke|
|European Junior Championships|
|Bronze||2010 Helsinki||400m freestyle|
|Gold||2011 Belgrade||400m freestyle|
|Gold||2011 Belgrade||50m breaststroke|
|Silver||2011 Belgrade||200m freestyle|
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.
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&M University. In her Freshman year she became the second-fastest A&M Aggie ever in the 100 metre breaststroke when she finished fourth in the event at the SEC Championships with a time of 59.35 seconds.
- Sycerika McMahon Wins European Junior Bronze Medal, Sport Northern Ireland
- European Junior Championships, Belgrade 2011, Swim Ireland
- Irish Olympic 2012 profile: Sycerika McMahon, SportsNews Ireland
- London 2012
- Women's 100m Breaststroke, London2012.com
- Women's 200m Individual Medley, London2012.com
- "2012 European Short Course Swimming Championships – Women's 50 metre breaststroke (final)". Omega Timing. Retrieved 2012-11-22.
- "Ireland’s Best Sycerika McMahon Commits to Texas A&M". Swimswam.com. Retrieved 20 September 2014.
- "Sycerika McMahon". Texas A&M website. Retrieved 20 September 2014.