The co-occurrence of mental illness and substance use disorders (SUDs) presents a significant public health challenge with affected individuals facing compounded stigma that leads to poor health outcomes, social exclusion, and systemic neglect. Despite growing recognition of stigma as a social determinant of health in people with comorbid mental illness and SUDs, current responses remain largely confined to clinical and academic settings. This article argues that civil society, particularly groups led by individuals with lived experience, represents an underutilized yet powerful force in combating stigma. Drawing from historical movements such as HIV/AIDS activism and contemporary examples from peer-led movements, we highlight how civil society organizations (CSOs) have reshaped public discourse, influenced policy, and fostered inclusive research. We examine emerging efforts in low resource settings and explore the transformative potential of digital civil society spaces. We advocate for a shift in stigma reduction paradigms to those that center lived experience, supports cross-sectoral collaboration, and recognizes both physical and digital civil society as essential to inclusive and sustainable change. To addressing the complex and intersecting stigmas associated with comorbid mental illness and SUDs, we recommend investing in CSOs, especially those grounded in participatory, culturally relevant approaches, particularly in low- and middle-income settings.