Goalkeeper Shannon was born in Canada to Scottish parents.
In 2009, in a bid to catch the eye of Scotland national coach Anna Signeul, she signed for Hibernian from semi-professional side FC Indiana, after a season where she conceded just two goals in the regular North American W-League.
She quickly earned a reputation as one of the best shot-stoppers in the Scottish Women's Premier League, and was voted Hibernian's Players Player of the Year for 2010. She was named Players Player of the Year at the 2011 Scottish Women’s Football Awards.
Shannon made her Scotland debut as a half-time substitute in a June 2010 friendly against Switzerland. She made a number of key saves in her first start for the national side, keeping a clean sheet in a 2-0 Cyprus Women's Cup win over England in March 2011, the first time that Scotland had defeated their old rivals for more than 30 years.