In the pursuit of carbon neutrality goals, local governments usually face the dilemma of whether to focus on economic development or environmental protection. Regional integration is a beneficial policy solution to address the challenges inherent to this dilemma. This research considers the expansion of the Urban Economic Coordination Committee in the Yangtze River Delta as a quasi-natural experiment in regional integration. It employs the difference-in-differences model to assess its potential for fostering a win–win situation in terms of economic growth and environmental protection, with the results showing that regional integration has a significantly positive impact on attaining this win–win situation. Inhibiting pollution transfer and promoting green transformation are crucial mechanisms by which regional integration can help strike a balance between economic growth and environmental protection. However, the effects of both regional integration and win–win mechanisms are heterogeneous across cities. The conclusion suggests that the government should accelerate the implementation of regional integration policies across a wider range, recognize and maximize the important role of intermediate mechanisms, and encourage cities to adopt different strategies according to their heterogeneous characteristics, forming a high-level collaborative development pattern.