Although born and raised in California, Karen played for England’s U19 and U21 teams before making her first senior appearance in the 2005 Algarve Cup against Northern Ireland. She had to wait four years for her next cap against Denmark, replacing the injured Rachel Brown in the Swindon friendly.
However, she soon staked her claim for the No.1 jersey and started all four matches at the FIFA Women's World Cup 2011. Karen made some important saves against New Zealand and France and comfortably saved Camille Abily's spot-kick in the quarter final defeat to the French.