This article explores how the appeal to depoliticized expertise worked to legitimize increased supervisory and executive power to the European Border and Coast Guard Agency, Frontex, after the 2015 refugee crisis. Frontex is an EU agency operating in a highly salient field, removed from hard‐science “gold standards” of evidence, where member states have been reluctant to delegate power and sovereignty. Through a process‐tracing case study, this article finds that appeals to technical neutrality, quantification, and objective indicators nevertheless were central when a new mandate for the agency was negotiated, giving Frontex unprecedented supervisory and executive power. They were also important resources for member states concerned about Frontex’s increased powers. By focusing on an agency at a remove from the natural‐science archetype, this article contributes to the literature on knowledge use in independent agencies. It suggests that technical expertise can be a powerful source of legitimacy even in a field removed from “hard” science.