Susan joined the Liberal Democrats in 1993 and in 2005 she was elected MP for her constituency of Richmond Park. During her time in the Commons she was a member of the Treasury Select Committee, Liberal Democrat spokesperson for International Development, then Trade & Industry, next Transport, followed by the Duchy of Lancaster and lastly for Heathrow.

Susan was introduced into the House of Lords in 2011 focusing on Finance and Business issues including her long-standing interest in developing a framework for community banks. She was appointed as Minister of State for Transport in October 2013. In 2015 Susan was appointed by Tim Farron as the Party Spokesperson for Economics.

Susan remains involved with a number of charities located in or with ties to Richmond as a Trustee of the Richmond Park Charitable Trust, a Trustee of the Kingston Trust and a Patron of the Holly Lodge Centre.