Background: Public safety personnel (PSP) such as paramedics and police are exposed to traumatic situations which increases their risk for mental health issues. However, many PSP do not seek help in a timely manner. Peer support interventions have the potential to decrease stigma and increase treatment-seeking behaviours among PSP. However, little is known regarding how the organizational culture of public safety organizations (PSOs) may affect the implementation of a peer-based intervention. This study aims to understand the extent to which organizational culture, including masculinity contest cultures (MCC), within Canadian PSOs affects implementation of PeerOnCall, a new peer support app for PSP. Methods: A multiple case study design was adopted, integrating semi-structured interviews and a standardized MCC scale completed with organizational champions from three PSOs. Two to three champions from each PSO acted as key informants regarding their organizations. Interviews explored champions’ perceptions of how organizational culture might shape implementation. Interview data were analyzed using inductive thematic analysis. Interview data were also triangulated with MCC scale data to understand the extent to which an MCC exists and is important to consider when implementing this app-based intervention. Results: Interviews with champions resulted in the construction of four themes. The first theme focused on external drivers of organizational culture shift, while the second theme focused on internal drivers. The third theme focused on how culture can create resistance to implementation. The fourth theme emphasized that culturally competent implementation strategies could overcome this resistance Most participants perceived their organization to have low-to-moderate levels of MCC. Discussion/Conclusions: Each organization had a unique and changing culture, as evidenced through its cultural artifacts, beliefs, and values. Understanding how culture shapes implementation of PeerOnCall can guide the creation of contextually relevant strategies that optimize implementation of PeerOnCall within PSOs. Recommendations for strategies and further study are provided. / Thesis / Master of Public Health (MPH) / This multi-site case study explores how first responder culture influences implementation of the PeerOnCall peer support app. Interviews and surveys were conducted with seven workplace champions from one police and two paramedic organization(s). Interviews explored organizational culture in general while surveys focused on masculinity contest culture specifically. It was observed that: external and internal drivers of culture shift have created a window of opportunity for PeerOnCall implementation; cultures of mistrust, skepticism & apathy, and hegemonic masculinity can create resistance to implementation; and culturally competent implementation strategies can overcome this. Traditional norms of masculinity appeared to be changing within the organizations. While public safety organizations’ cultures have been studied, this study links culture to the implementation of a specific intervention and offers nuance to the issue of masculinity contest cultures. Findings can inform the creation of culturally competent strategies that optimize the implementation of resources such as PeerOnCall among first responders.