In 1977, Cartmail began to work as a trade union liaison officer for the London Borough of Greenwich, and ten years later became a full-time trade union official for the Health Visitors' Association. She left in 1990 to become a regional officer for the Manufacturing, Science and Finance union, being promoted to national officer before it became part of Amicus, in which she was the national officer for the health sector.[2]
In 2007, Amicus became part of Unite, and Cartmail was appointed as its assistant general secretary with responsibility for public services.[2] She contested the 2010 election to become the union's General Secretary, taking fourth place and 39,000 votes behind Len McCluskey who was elected.[3] In 2016, when McCluskey stood down to contest a new election, Cartmail was chosen as acting general secretary.[4]