We examined the effects of individual versus joint action and racial similarity and dissimilarity on a Simon task using mouse tracking to explore the implicit cognitive dynamics underlying responses. Participants were slower to respond when working with a partner than when working alone, and their mouse movements also differed across conditions. Participants paired with a different-race partner took longer to respond than participants paired with a same-race partner. We argue that, in the joint conditions, participants’ longer responses were the result of automatic inhibitory processes that arise within the social context.