Objective Emergency service workers are at risk of experiencing poor mental health due to repeated exposure to potentially traumatic events. Promoting healthy lifestyle factors may help improve health outcomes and quality of life among this population. This study aimed to evaluate the efficacy of a 10-week physical activity (PA) and diet programme delivered via Facebook for sedentary emergency service workers and their support partners on levels of psychological distress. Methods We delivered a 10-week 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 PA tracking device (Fitbit accelerometer). A stepped-wedge design was applied to compare levels of psychological distress (K6) during baseline, to intervention by comparing slopes of change. Secondary pre–post outcomes included mental health symptoms, PA, quality of life, social support to exercise, sleep quality and suicidal ideation. Results In total, N=90 participants ( n=47 emergency service workers and n=43 support partners) were recruited in 4 cohorts (aged 42.3±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 slopes were significantly different, b=−0.351, p = 0.003 (i.e. the trajectories of change) and improvements plateaued until follow up. Retention was high (92%) and improvements in mental health symptoms, minutes of PA, sedentary time and quality of life were significant. Conclusions Our intervention delivered via social media is feasible and associated with reduced levels of psychological distress among emergency service workers and support partners. Trial registration Australian New Zealand Clinical Trials Registry (ACTRN): 12619000877189.