Background Rates of anxiety and depression have been increasing among children and adolescents for the past decade; however, many young people do not receive adequate mental health care. Digital mental health interventions (DMHIs) that include web-based behavioral health coaching are widely accessible and can confer significant improvements in youth anxiety and depressive symptoms. However, more research is necessary to determine the number of web-based coaching sessions that confer clinically significant improvements in anxiety and depressive symptoms in youth. Objective This study uses data from a pediatric DMHI to explore the number of web-based coaching sessions required to confer symptom improvements among children and adolescents with moderate or moderately severe symptoms of anxiety and depression. Methods We used retrospective data from a pediatric DMHI that offered web-based behavioral health coaching in tandem with self-guided access to asynchronous chat with practitioners, digital mental health resources, and web-based mental health symptom assessments. Children and adolescents who engaged in 3 or more sessions of exclusive behavioral health coaching for moderate to moderately severe symptoms of anxiety (n=66) and depression (n=59) were included in the analyses. Analyses explored whether participants showed reliable change (a decrease in symptom scores that exceeds a clinically established threshold) and stable reliable change (at least 2 successive assessments of reliable change). Kaplan-Meier survival analyses were performed to determine the median number of coaching sessions when the first reliable change and stable reliable change occurred for anxiety and depressive symptoms. Results Reliable change in anxiety symptoms was observed after a median of 2 (95% CI 2-3) sessions, and stable reliable change in anxiety symptoms was observed after a median of 6 (95% CI 5-8) sessions. A reliable change in depressive symptoms was observed after a median of 2 (95% CI 1-3) sessions, and a stable reliable change in depressive symptoms was observed after a median of 6 (95% CI 5-7) sessions. Children improved 1-2 sessions earlier than adolescents. Conclusions Findings from this study will inform caregivers and youth seeking mental health care by characterizing the typical time frame in which current participants show improvements in symptoms. Moreover, by suggesting that meaningful symptom improvement can occur within a relatively short time frame, these results bolster the growing body of research that indicates web-based behavioral health coaching is an effective form of mental health care for young people.