Mental health is one of the most important contributors to the global burden of disease in children and adolescents. Mental health conditions are associated with lower quality of life in adulthood. These conditions have an early onset and typically first occur in childhood. However, little is known about how these conditions are related to service utilization before the initial diagnosis, or about the significance of the diagnosis on later service utilization. To study this, register data on 5-15-year-old children in the city of Oulu, Finland, covering the years 2013–2018 were used. To identify the association of mental health and behavioral conditions with service utilization, counterfactuals were constructed from children who were similarly diagnosed three years later. Event study regressions on several health care and service utilization outcomes were estimated. The findings showed that primary and specialized health care utilization increased sharply before the initial diagnosis and peaked during the time of diagnosis. Primary care utilization started decreasing slowly after, while specialized health care utilization remained high for two years after the diagnosis. Probability of visiting a mental health professional, use of rehabilitation services, psychiatric medication, and utilization of child protection services increased significantly after the diagnosis. The results highlighted the importance of outpatient health care in detecting and treating the conditions in children. In a fragmented system, knowledge on utilization trajectories in different services may be of help in allocating resources to improve the health of these children.