The outcomes of new product development (NPD) processes are dependent on the interplay of several interdependent activities. One product development activity can be dependent on the presence or absence of other activities, different kinds of NDP processes may lead to the same outcome, and specific kinds of activities may have a positive effect in one process but no effect in other processes. However, we currently lack means to examine and explain this causal complexity inherent in NPD processes. To address this issue, we introduce mechanism-based approach as a way to capture conjunctural and equifinal causal relations. We build this approach on the philosophical literature on mechanism-based explanations and the methodological opportunities provided by the qualitative comparative analysis (QCA) to identify how the activities of entities are configured together to generate outcomes. We elaborate this approach by presenting an in-depth historical analysis of the NPD projects of Vaisala, a meteorological instrument company. We discover and suggest that the company’s NPD projects were driven by three mechanisms (ideation, evaluation and commercialisation) and that each of them were actualised by a set of different activity configurations. Accordingly, we contribute to the NPD and innovation literature by showing how mechanism-based explanations take into account both the abstract theorisation of NPD processes and their inherent causal complexity.