Mary was elected onto the Usdaw Executive in 2000 and later became a member of the STUC General Council.

She stepped down as a councillor for the Renfrew South and Gallowhill Ward at the 2012 Scottish Local elections.

Under the leadership of Johann Lamont, Fee was Scottish Labour's Shadow Housing Minister; then under Jim Murphy, Fee was promoted to the position of, Spokesperson for Infrastructure, Investment and Cities before later taking on the position of Spokesperson for Reform, after Kezia Dugdale was elected as the party's new leader in August 2015.

In September 2015, Fee's amendment to the Criminal Justice (Scotland) Bill was passed after receiving cross-party support.

[4] Fee nominated Anas Sarwar in the 2021 Scottish Labour leadership election.