When faced with complex social and environmental issues, some organizations participate in Cross-Sector Social Partnerships (CSSPs) to attempt to improve the issue through collaboration. Some of these CSSPs are facilitated by formal convening organizations that enable the active participation of stakeholders. This research study contributes to our understanding of the role of leadership in these organizations by exploring the question: what are the intentional leadership activities of convening organizations in the formation of CSSPs? Aligned with the process approach to leadership represented in Relational Leadership Theory, narratives drawn from a variety of CSSP stakeholders are interpreted as social constructions of the intentional leadership activities of convening organizations in the formation of CSSPs. Data was collected from seven CSSP research sites that were in their formation stage and were facilitated by enterprising convening organizations. Data gathering methods included conducting semi-structured interviews, engaging in participant observation and collecting organizational documents. A rigorous process of data interpretation and analysis was undertaken and theoretical coding was informed by insights from Social Movement Theory (SMT). Findings indicate that the intentional leadership activities of convening organizations involve three broad categories: i) identifying political opportunity; ii) creating mobilizing structures and; iii) engaging in framing activities. These activities are conceptualized in terms of a new construct called ‘mobilizing leadership’ that is associated with the fostering of collective action of stakeholders in the CSSP. Mobilizing leadership involves opportunity recognition and creation, which is linked to the mobilization of stakeholders’ tangible and intangible resources without the expectation of monetary reciprocity. The contributions of this research are both theoretical and empirical. Concepts from the SMT literature were used to further develop themes on the leadership of convening organizations inductively identified through empirical analysis. This grounded interpretation of leadership at the formation stages of CSSPs is intended to initiate a scholarly discourse related to ‘mobilizing leadership’ in the CSSP literature. The research findings also constitute a novel empirical examination of intentional leadership activities in the formation of CSSPs that are facilitated by enterprising convening organizations. Practitioners may benefit from this as it will inform a deeper understanding of the intentional leadership activities they need to engage in to catalyze the formation of CSSPs. These insights may be particularly useful to those practitioners interested in mobilizing the collective action of stakeholders in CSSPs in the context of limited financial resources.