After starting her career at Conwy Devils, Foster moved to Bangor City and was the club's Northern Division top-scorer in every season from 1999–00 to 2002–03. She also represented the club in the UEFA Women's Cup. In summer 2003 Foster joined an exodus of players leaving Bangor City for Liverpool. By season 2009–10, she was Liverpool Ladies' longest serving player. That season she was the Northern Division top goalscorer with 16 goals as Liverpool romped to the title. Foster and Liverpool teammate Kelly Jones joined FAI Women's Cup winners St Francis on loan for a 2010–11 UEFA Women's Champions Leaguequalifying tournament in August 2010. Foster had planned to retire at the age of 30 years, but changed her mind in order to play in the FA WSL. She then scored Liverpool's first goal in the new competition, helping them to a 3–3 opening day draw against local rivals Everton. In October 2012 Foster was amongst ten players to be released by Liverpool. She signed for Doncaster Rovers Belles in January 2013. During the 2013 mid-season break Foster left Doncaster after struggling to balance the demands of travel and training with her career outside football. She announced her intention to start coaching and that she would consider whether to continue playing.
International career
Foster represented Wales at youth level, winning five U16 caps, four U18 caps and scoring one goal in a single U20 appearance. Her full debut was in 1997 and she captained the side on her 50th appearance, a 3–0 friendly win over Slovenia in Llanelli in August 2009. Foster won a record 52nd cap in Wales' 2–1 defeat in Azerbaijan in October 2009. In May 2014, the Football Association of Wales recognised Foster's 63 appearances with the award of a golden cap at half-time of Wales' 4–0 win over Montenegro in Bangor.
Refereeing
After deciding not to extend her career as a player in 2013, Foster instead started training as a referee. She was an assistant referee at the 2013–14 Welsh Women's Cup final. In December 2015, during her third season as a referee, Foster was named on the FIFA international list. On 18 August 2018, she became the first female referee to take charge of a Welsh Premier League game.