Cartmail attended Heronswood Secondary Modern School in Welwyn Garden City, having failed her eleven plus exam. She left school with no qualifications, Cartmail began her working life as an apprentice hairdresser. Later, she began working in the publishing industry, and eventually became Mother of the Chapel for the print union, the National Graphical Association, where she gained prominence for a campaign for equal pay for women, and also spent time studying at the London College of Printing.
In 1977, Cartmail moved 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.
In 2003, Amicus became part of Unite, and Cartmail was appointed as its assistant general secretary with responsibility for public services. She contested the 2010 election to become the union's General Secretary, taking fourth place and 39,000 votes, Len McCluskey winning the post. In 2016, when McCluskey stood down to contest a new election, Cartmail was chosen as acting general secretary.
- McSmith, Andy (2010-05-31). "The woman who wants to end male dominance – and unite Unite". The Independent. Retrieved 2010-06-27.
- Mulholland, Hélène (21 July 2010). "The 'edgy' candidate making a bid for Unite's top job". The Guardian. Retrieved 8 December 2016.
- "Len McCluskey chosen as new Unite leader". BBC News. 21 November 2010.
- Syal, Rajeev (6 December 2016). "Len McCluskey resigns as head of Unite but will run again for leadership". The Guardian. Retrieved 8 December 2016.
- "Gail Cartmail". gov.uk. Retrieved 23 October 2016.