Like in other countries with neoliberal industrial relations (IR) regimes, unionization in Chile declined throughout the 1990s and 2000s. However, in the last decades, this declining trend was interrupted. Between 2007 and 2020, the union density rate increased from 13% to 19%. This rise in union membership is puzzling because no significant pro-labour reform was implemented in this period. Using panel data on Chile’s nine main economic industries (1999–2019), in this article the authors show that the rise in strike activity was a fundamental stimulus for the growth of union membership. Additionally, using mediation analysis on individual-level cross-sectional data (2014–2018), the authors explore micro-level mechanisms that can explain the relationship between strike participation and union membership. They find that the workers who are more willing to strike are more likely to unionize because they trust unions more and especially because they are more politicized.