ObjectiveEmergency service workers are at risk of experiencing poor mental health due to repeated exposure to potentially traumatic events. Promoting physical activity and diet may help to mitigate some the consequences of emergency work and so this study aimed to evaluate the impact a lifestyle intervention on levels of psychological distress among emergency service workers and their support partners.MethodsWe delivered a 10-week physical activity intervention via a private Facebook group facilitated by exercise physiologists, a dietitian and peer-facilitators. Weekly education modules and telehealth calls were delivered, and participants were provided with a physical activity tracking device. A stepped-wedge design was applied to compare levels of psychological distress (Kessler-6) during baseline, to intervention by comparing slopes of change. Secondary pre-post outcomes included mental health symptoms, physical activity, quality of life, social support to exercise, sleep quality and suicidal ideation.ResultsN=90 participants (n=47 emergency service workers and n=43 support partners) were recruited in 4 separate cohorts (mean age 42.3(SD=11.5) years, 51% male). Levels of psychological distress did not change significantly during the baseline (control) slope and reduced significantly during the first 6 weeks of intervention (intervention slope 1). The interaction between slopes were significant, b=-0.351, p = 0.003, (i.e., the trajectories of change were significantly different) and improvements plateaued until the 4-week follow-up. Retention was high (92%) and improvements in mental health symptoms, minutes of physical activity, sedentary time and quality of life were observed.ConclusionsA physical activity intervention delivered via social media is effective in improving psychological distress among emergency service workers and support partners.Trial registrationAustralian New Zealand Clinical Trials Registry (ACTRN): 12619000877189.