Amid the ongoing green and low-carbon transition, environmental non-governmental organizations (ENGO) have become increasingly influential as one of the informal regulatory actors driving corporate emissions reductions. This paper utilizes panel data from Chinese A-share listed companies over the period 2000-2022 to examine the effect of informal environmental regulation on corporate carbon emission performance and the underlying mechanisms. The analysis is based on a precise measure of informal environmental regulation at the firm level and employs robust econometric techniques, including a two-way fixed effects model and an instrumental variables approach. The results show that informal environmental regulation significantly improves corporate carbon emission performance, with the findings remaining robust across a series of sensitivity checks and after addressing potential endogeneity issues. Heterogeneity analysis shows that the positive impact of informal environmental regulation is more significant in firms with CEOs who have green experience, state-owned firms, non-heavy-polluting firms, as well as in firms located in central and western China and non-resource-based cities. Mechanism analysis shows that the observed effect operates primarily through increased media attention, the promotion of green technological innovations, and the strengthening of formal environmental regulations.