Mass gatherings pose unique challenges for interorganizational collaboration. The "Athens Marathon, The Authentic" is a 42,195m (26.2mile) race with approximately 18,000 runners that increases annually. On the same day, additional races take place and a grand total of more than 50,000 runners fill the city center of Athens, Greece. Responding effectively to unexpected incidents requires comprehensive planning, clear decision-making structure, and effective collaboration. Nonetheless, there is limited empirical evidence to support interagency collaboration in mass gatherings.This study used the 2017 Athens Marathon and related races as the empirical setting to examine how interagency collaboration was perceived among the multiple public health and safety professionals involved in the marathon command center.Data comprised 10 semi-structured, in-depth interviews with key informants, direct observations of meetings and the event itself, and documentary analysis. Open coding and thematic analysis were used to analyze the data.Findings indicated four key components of interagency collaboration in such an event: organizational culture, team synthesis, on-site spatial planning, and the usage of radio-amateurs.This study outlined the factors that shaped interagency collaboration in the context of a mass event. Practical implications arising from this study may inform the ways organizers of marathons and other mass sporting events can engage in effective partnerships and joint working.