Despite the increasing deployment of formalized boundary spanning roles and practices, the mechanisms and dynamics of their legitimation remain under-explored. Using the Bourdieusian lens, we theorize legitimation of boundary spanning as accumulation, mobilization and conversion of several forms of capital unfolding in a configuration of intersecting fields. Drawing on a qualitative longitudinal case study of a collaborative partnership between a university and healthcare organizations, we describe changes in the structure, sources and mutual convertibility of capital assets over time. We also analyse the implications of this evolution for the relationships between the intersecting fields and the social trajectory of boundary spanners. We argue that legitimation of boundary spanning roles and practices is a highly transformative, collective and political process that increases the capital endowments and authority of individual boundary spanning agents but may lead to the erosion of the very same roles and practices that were being legitimized.