Extensive evidence from cross-sectional studies has revealed a positive link between greenspace access and walking behaviors. However, the inherent weaknesses of the cross-sectional research design have provided little causal inference. In this natural experimental study, we assessed the effects of a large-scale greenway intervention (i.e., the opening of East Lake greenway) on walking behaviors in Wuhan, China. Longitudinal survey data on 1,020 participants were collected before and after the intervention in 2016 and 2019, respectively. The results of the mixed-effect difference-in-difference (DID) models showed that the greenway intervention had a significantly positive effect on the walking time, especially for residents living within two kilometers from the greenway. Furthermore, women and socio-economically disadvantaged people benefited most from the greenway implementation regarding walking time. Our findings provided compelling evidence that public investment in transportation infrastructure (e.g., greenway) effectively promotes walking behaviors and mitigates social inequities in physical activity.