BACKGROUND: Emergency service workers are at an elevated risk for stressor-related mental health (MH) issues, such as anxiety, depression, and posttraumatic stress disorder. Barriers to help-seeking are widespread across this sector, necessitating interventions tailored to the unique needs of this population. Build Back Better is a smartphone-based intervention designed to provide evidence-based strategies for the prevention of anxiety, depression, and posttraumatic stress disorder among emergency service workers. OBJECTIVE: This study aimed to evaluate the usability, acceptability, feasibility, and preliminary effectiveness of the Build Back Better app among emergency service workers. METHODS: A single-group (N=67), 1-month pilot study assessing the impact of the Build Back Better app on MH outcomes, including general distress, anxiety, depression, and traumatic stress coping, was undertaken with emergency service workers. Participants completed baseline and 1-month follow-up assessments using the Kessler Psychological Distress Scale, 9-item Patient Health Questionnaire, 7-item Generalized Anxiety Disorder, World Health Organization Well-Being Index, and the Trauma Coping Self-Efficacy Scale. The app's usability and acceptability were also evaluated through participant feedback and usage data. RESULTS: Of the 71 participants enrolled, 67 completed the baseline assessment and downloaded the app, with 33 participants providing follow-up data. The mean age of participants was 44.73 (SD 11.4) years, and 64% (n=43) were male. The majority of respondents rated the app quality as very high (n=27, 79%), felt that the app was easy to use (n=20, 61%), easily understood (n=18, 55%), improved their mental fitness (n=27, 80%), and would recommend the app to others (n=20, 61%). Encouraging trends toward improvement were found across symptom and well-being outcomes; however, these trends were not significant: general distress (t32=0.65, P=.52), depression (t32=0.75, P=.46), anxiety (t32=1.08, P=.29), or traumatic stress coping (t32=-0.27, P=.79), with effect sizes ≤0.2, likely due to the small sample size. CONCLUSIONS: The Build Back Better app demonstrated satisfactory levels of usability and acceptability. While the pilot study showed encouraging trends toward improved MH, further research with a larger sample size is needed to determine its efficacy. Participants furthermore suggested improvements in app navigation and content clarity, emphasizing the need for a more intuitive user experience. Given the positive feedback and improvement in MH outcomes, a larger-scale efficacy trial is warranted to further assess the app's potential for MH support in this high-risk population.