BACKGROUND: Variability in expertise and risk tolerance among emergency departments (ED) clinicians, when assessing and managing pediatric mental health presentations, leads to increased resource utilization. HEARTSMAP is a validated electronic tool that supports ED clinicians in psychosocial assessments and disposition decision making. METHODS: We used interrupted time series analysis (September 2016-December 2019) and multivariable regressions to measure the impact of integrating HEARTSMAP into ED practice on pediatric mental health presentations length of stay and return visits, at two pediatric EDs. The intervention site used HEARTSMAP trained ED clinicians to assess and manage mental health presentations, and reported bi-weekly ED median length of stay and 30 days-return visits for 15 months and a year, during passive and active implementation of HEARTSMAP, respectively. The control site used psychiatric nurses to assess and manage patients and was only exposed to passive implementation. RESULTS: HEARTSMAP average uptake was on average 47.4% (range 23.8-74.6%) during active implementation at the intervention site, while the control site showed no uptake throughout the study period. Incremental HEARSTMAP (each percent increase) use was associated with a reduction of 1.8 min (95% CI 0.8-2.9 in ED length of stay and 0.3% (95% CI 0.2-0.5 in 30-day return visit rate. This translates to an adjusted average reduction of 85.3 min in ED length of stay and 15.2% in 30-day return visits for youth with mental health presentations. CONCLUSION: Use of HEARTSMAP in the ED can decrease length of stay and return visits for emergency pediatric mental health visits, in a fixed-resource setting.