Public administration research has recently paid increasing attention to public employees’ social categorization of citizens and the consequences thereof for administrative decision-making. We advance this line of scholarship by theorizing the concept of implicit citizenship theories (ICTs) and elaborating it in four sequential empirical studies. ICTs are implicit assumptions about citizens’ typical or ideal characteristics that emerge freely and associatively in the minds of public employees and guide cognitive categorization. We investigate the content and structure of these ICTs and extract a taxonomy of six distinctive, yet interrelated, prototypes of citizens that public employees bring to their jobs. As implicit theories influence attitudes and behaviors, ICTs are relevant to the quality of public services delivered to citizens. This is particularly evident where negative prototypes give rise to unequal treatment. Practical interventions to change those prototypes might target their individual beholders or the social context that makes ICTs accessible.