After playing for Doncaster Belles at Under–18 level,[2] Johnstone moved to Scotland in February 2001, where she was signed for Kilmarnock by manager Jim Chapman.

[3] In the 2004 summer season, Johnstone played in Iceland with ÍBV and made 13 Úrvalsdeild appearances.

[5] In October 2010 Johnstone played as a substitute striker due to an injury crisis and scored in Glasgow City's 16–0 win over Aberdeen.

[6] Before the 2012 Scottish Women's Premier League season, Johnstone held transfer talks with Forfar Farmington.

[8] Johnstone made her senior Scottish national team debut against Moldova in April 1999.