This article examines the impact of England's first women justices of the peace (JPs) on the work of lay magistrates in the period 1920–50. It argues that the early women JPs (many of whom had been active in the women's suffrage campaign), and the organizations that they belonged to, helped to transform the institution of the lay magistracy by adopting a more ‘professional’ approach, evidenced in their willingness to educate and train themselves for their new role. In consequence, this article challenges conventional definitions of ‘professionalism’, arguing that, where the work of JPs was concerned, the boundary between ‘voluntary’ and ‘professional’ activities was less clear than might be supposed. Furthermore, the willingness of many women magistrates in particular (later followed by some of their male colleagues) to undergo training helped to ensure the survival of the lay element in the criminal justice system to the present day.