Workplace mental health is a significant and growing issue for many employers. According to the World Health organization, an estimated 12 billion working days are lost every year to depression and anxiety at a cost of $1 trillion per year in lost productivity. Mobile health technology has the potential to provide scalable, accessible, customizable supports that can reach a wide range of employees, providing information and support when and where it is needed. There is a growing number of mental health apps and emerging evidence on the value of apps in the workplace, but there is also wide variation in adoption and impact. This presentation will focus on implementation science as an approach to evaluating implementation of PeerOnCall, a new mobile app designed by and for Canadian first responders (police, fire, paramedics, correctional workers, 911 operators). The app includes links to ‘peer wisdom’ videos, self-management tools and local resources as well as an option to make a secure, anonymous connection to trained peer support providers via phone or text. Research was conducted over a 4-6 month period in 42 diverse first responder organizations across Canada. We will highlight opportunities and challenges in the implementation process and how this affected engagement and adoption of the technology within and across sectors. Key recommendations include the need for culturally-relevant implementation strategies in order to build credibility, trust and the technology and peer support infrastructure required for success. We will also highlight the role of mobile health technology as a ‘tool in the toolbox’ and the need to embed this tool within a stepped model of workplace mental health support. This research also contributes valuable insights into ‘what works for whom in what context’, highlighting the importance of research that combines a focus on implementation as well as impact. Note: This presentation is a synthesis of research projects funded by CIHR, Movember, and the Public Health Agency of Canada.