Significant public discourse has focused recently on police–civilian interactions involving with persons with mental illness (PMI). Despite increasing public attention, and growing demands for policy change, little is actually known about the myriad of ways in which Canadian police encounter PMI in the context of routine police work. To assist policymakers in developing evidence-informed policy, this paper attempts to shed light on present difficulties associated with addressing fundamental questions, such as the prevalence of mental health related issues in police calls for service. To do this, we attempt to map the size and scope of police calls for service involving PMI, drawing on both the available scientific data and the limited knowledge to be gleaned from available police reports. Our focus is on two broad categories of police interactions with citizens: public safety concerns (wellness checks, suicide threats, missing persons, mental health apprehensions) and crime prevention and response (encountering PMI as victims–complainants and (or) as potential suspects). We also explore the challenges policy-makers face in relying on police data and the importance of overcoming weaknesses in data collection and sharing in relation to the policing of uniquely vulnerable groups. This paper concludes with some key recommendations for addressing gaps highlighted.