This article develops a concept of social brokerage to explain leftist–Sadrist cooperation during Iraq's 2015 protest movement. Conventional understanding holds that Iraq's secular-leftist civil trend and Shiʿi Islamist factions have been mutually isolated, and at times fierce antagonists, in Iraq's post-2003 politics. This view has been challenged by an emergent political alliance between a faction of the civil trend and the Shiʿi Islamist Sadrist movement. By comparing this alliance with the failure of another Shiʿi Islamist group, ʿAsaʾib Ahl al-Haq, to involve itself with and exploit the protest movement, this article isolates the conditions which determined the dynamics of leftist–Islamist interactions. Shifting the focus away from elite politics and structural-instrumental explanations favored by rational choice models, this article reveals a longer backstory of social and ideological interactions between less senior actors that transgressed leftist–Islamist social boundaries. From this context, potential brokers emerged, capable of skilfully mediating leftist–Sadrist interactions.