The Gilded Age was a tumultuous period for US labor and capital: the labor movement grew in size and intensity, strikes mushroomed, and women’s labor force and strike participation grew in tandem. Yet little is known about how women’s participation influenced strike efficacy. On the one hand, women may have added numerical force, militant energy for gendered solidarity and therefore contributed to strike success. On the other hand, women’s participation may have hindered the cause by delegitimizing it or by producing harmful internal factionalism. In this article, we ask: How did women’s participation in Gilded Age strikes influence strike success? We use a unique data source to test the impact of women’s participation relative to men on the success of every strike that took place in the Northeastern United States from 1881 to 1886. We find that striking gender composition ratios matter and are reflected in a nonlinear pattern: for male-dominated and female-dominated strikes, greater numbers of female strikers reduce the chances of success. However, when gender composition approaches approximate parity, the effect of female strikers enhances strike success. We suggest that in approximate proportional parity range, women were more likely to participate and also be taken more seriously by male co-workers. We supplement quantitative findings with qualitative accounts from specific strikes. Our findings have important implications for diversity in contentious collective action in general and strike success in particular.