China’s Environmental Protection Tax (EPT), introduced in 2018, provides a useful setting for examining whether pollution-oriented regulation can also deliver carbon-mitigation benefits and whether such benefits depend on prior low-carbon policy exposure. Using panel data for 285 Chinese prefecture-level cities from 2002 to 2022, this study first estimates a difference-in-differences model exploiting cross-provincial variation in the post-2018 EPT shock and then applies a triple-difference framework to examine whether prior exposure to the Low-Carbon City (LCC) and Carbon Emissions Trading (CET) pilots was associated with stronger EPT effects. The results show that, on average, the EPT reduced PM2.5 concentration, carbon emissions, and carbon intensity by approximately 3.7%, 9.6%, and 10.8%, respectively, although the evidence is stronger for the two carbon-related outcomes than for PM2.5. The clearest and most stable heterogeneous evidence appears for carbon intensity, especially for the LCC-only group; the Dual group shows the largest point estimate, but its external validity is limited. Further analysis suggests that post-EPT changes in industrial structure upgrading and green invention patent grants were more visible in cities with prior low-carbon policy exposure. Overall, the findings indicate that prior low-carbon policy exposure was associated primarily with a stronger EPT effect on carbon intensity.