 Mills worked in legal publishing, where she joined the National Society of Operative Printers and Assistants.She was elected as Mother of the Chapel, representing other trade unionists at her workplace, before taking a full-time post with the Society of Graphical and Allied Trades.In 1985, she moved to work for the National Union of Public Employees (NUPE) as an Area Officer in the London Division, before being promoted to a National Officer post responsible for Equalities work within the union.