The 21st century has borne witness to catastrophic natural and human-induced tragedies. These disasters necessitate humanitarian responses; however, the individual and collective bases of support are not well understood. Drawing on Duncan’s motivational model of collective action, we focus on how individual differences position a person to adopt group memberships and develop a “group consciousness” that provides the basis for humanitarian action. Longitudinal mediation analyses involving supporters of international humanitarian action (N = 384) sampled annually for 3 years provided support for the hypothesized model, with some twists. The results revealed that within time point, a set of individual differences (together, the “pro-social orientation”) promoted a humanitarian group consciousness that, in turn, facilitated collective action. However, longitudinally, there was evidence that a more general pro-social orientation undermined subsequent identification with, and engagement in, the humanitarian cause. Results are discussed in terms of understanding the interplay between individual and group in collective actions.