The race was won by the Czech Republic’s Petra Chocova in 31.25 with Germany’s Caroline Rahnau taking the bronze medal.
McMahon now joins Gary O’Toole (1989 Bonn) Michelle Smith (Vienna 1995 and Seville 1997) and Gráinne Murphy (Budapest 2010) as a medallist at a European Long Course (50 metres) Championship meet.
The 17-year-old Belfast teenager eased her way through the heats and semi-final on Saturday to emerge into the last eight as the fastest qualifier through to the final.
McMahon won her heat and was second in her semi-final in a time of 31.65 and was in prime position to take at least a bronze.
However, in the final, the former double junior European gold medallist put in the swim of her life to claim a silver medal, touching home in a blanket finish in a time of 31.27.
In fact she just missed out on the gold medal by .02 of a second.
McMahon also broke her Irish senior record of 31.49 which she set at the World Championships in Shanghai.
McMahon has had a seamless transition into the senior ranks and secured Olympic qualification last Tuesday in the 100m breaststroke final, where she finished fifth in the final.
“It’s been a great week for Irish swimming,” said Peter Banks, Swim Ireland’s high performance director, who had hoped to have six individual swimmers at the 2012 games.
“Two FINA ‘A’ standards earlier in the week and then to top it off Sycerika’s great performance for the silver medal, the team have had a great championship overall,” said Banks.
The championships proved very successful for the Irish with four finalists, eight semi-finalists, three new Irish Senior records, two Olympicqualifications and a silver medal as McMahon and Nocher join Barry and Gráinne Murphy as Ireland’s representatives in London.
Irelands Women’s 4x100m Medley Relay team of McMahon, Melanie Nocher, Bethany Carson and Fiona Doyle swam the heats yesterday and finished 12th overall in a time of 4:13.75, a second and a half outside the Irish record
The cut off date for Olympic qualification is May 31 but Chris Bryan has one last chance to make the 10K swim at London when he contests the final qualifier in Portugal next month.