In a sample of young people in Northern Ireland (N = 819), we examine the relationships between the quality of experience with police officers and police legitimacy. We examine potential pathways through which experiences may either support or undermine the legitimacy of the police, and thus cooperation and compliance with them. We find evidence that perceptions of the police as having goals that align with those of wider society, and as being fair in general, mediate relations between the quality of encounters and legitimacy, which in turn mediates the relation with cooperation and compliance. Identification with wider society was not a reliable mediator, contrary to our predictions based on the Group Engagement Model. Moreover, our analysis of the structure of police fairness perceptions finds no support for the distinction between procedural and distributive police fairness as usually conceived. Implications for the social psychological understanding of legitimate authority are discussed.