Intermediaries are social agents who can be found in all types of different environments, cultures, and organizations. More often than enough, intermediaries are middlepersons between two power centers, yet their agency is precarious due to their position as brokers who gain from bridging otherwise unconnected parties, or marginalized vulnerable individuals who suffer from the invasion of the neighboring power holders. How can these two different perspectives of the intermediaries be reconciled? Under what conditions do they shift from one type of (...) agency to another? Previous scholarship attributes the agency of intermediaries to static positionality. This paper treats the position of the intermediary as an opportunity structure, which is shaped by multi-level relational dynamics. We show the changing agency of three intermediary tribal groups (i.e. the Tibetans in northwestern China, Uriyangqad Mongols, and Jurchens) who were situated between the sedentary Chinese and nomadic Mongols during the Ming dynasty (1368–1644) through a systematic study with horse trade data. The group-based and officially-regulated horse trade constituted the most important linkage between the intermediaries and these two power centers. We study the changes in the dynamics of their contact during trade, and explain why these intermediaries became client, client to two patrons, and broker in their trade with the Chinese and Mongols in different eras. We argue that the position of the intermediaries did not provide definite advantages for their exploitation, nor did it paralyze them with any concrete disadvantages. Rather, their position offered them unpredictable opportunities that arose from their relational dynamics with the power centers. Moreover, the self-serving and local interests of the intermediaries, and the strategies and tactics that they used, determined their capacity to respond to emerging and unpredictable opportunities and capitalize on them. (shrink)