Scholarship abounds on the importance of city-regions to regional and national prosperity, and to the wider global economy. But little is known about their capacity to function as effective, legitimate and robust policy actors. This article begins to address the important question of what determines the governance capacity of city-regions by unpacking the concepts at the core of this research. It focuses on sources of horizontal capacity as a function of the strength of intermunicipal partnerships. Research suggests a variety of determinants of the strength of inter municipal partnerships, from rational choice to institutional perspectives. This article acknowledges the contribution of these approaches, but argues that none of the approaches presented to date can alone explain observed variations in the strength and capacity of city-regional partnerships. Instead the article presents an alternative theoretical framework that reimagines and combines existing approaches, and introduces the concept of civic capital as a critical determinant of governance capacity.