Navigating political action typically requires coalition building across different interest groups. Because members represent divergent interests and are embedded in different networks, however, alliance formation is difficult. In this article, I consider how elites rely on political brokerage to overcome these divisions and form successful coalitions, with successful organization of the parliamentary opposition to the king before the English Civil War as a case in point. Supporting quantitative evidence comes from rich network data on the business, kinship, and political affiliations among 346 political and mercantile elites. Rather than the mere presence of mediator positions, I argue that effective brokerage of mobilizing alliances between interest groups requires political mediators who are equally affiliated with these diverse networks. Successful brokerage is conditional on both their structural position between groups and the diversity of ties that compose their personal networks. The results demonstrate that new merchant elites who were engaged in the American colonial trades acted as political mediators and facilitated the formation of a parliamentary opposition strong enough to defeat the royalist forces in London.