This article explores the initial reasons for union joining of women who became union leaders in the UK and the USA by drawing on concepts from mobilization theory and the literature on women and unions. The comparative study demonstrates similarities and differences in early mobilization influences on UK and US women with respect to family, ideology, instrumentality and injustice. Informed by the women and unions literature, the article critiques mobilization theorists for failing to problematize the term ‘injustice’ and underplaying the importance of ideology which are shown to be gendered and racialized and located in time and place.