Background The COVID-19 pandemic introduced complexities that were likely more demanding for some groups, such as children and adolescents, and especially those with pre-existing mental health diagnoses. This study examines long-term patterns of psychiatric healthcare utilization among this vulnerable group, providing insights into shifts in psychiatric healthcare use during a global health crisis. Methods We use data from the primary and specialist healthcare registries available from the Norwegian emergency preparedness register for COVID-19 (Beredt C19) to estimate patterns of psychiatric healthcare utilization. The data spans the years from 2017 to 2022, covering children and adolescents aged 6–19. We identified young people with recent diagnoses of mental health conditions and compared weekly consultation volumes before and during the pandemic for mental health conditions overall and the following diagnostic categories separately: anxiety/depression, Attention-Deficit Hyperactivity Disorders (ADHD), sleep problems, and Autism Spectrum Diagnoses (ASD). Analyses were stratified by gender. Results There was a consistent trend of lower-than-predicted weekly healthcare consultations among young people with pre-existing mental health diagnoses in both primary and specialist healthcare during the pandemic. The reduction was more pronounced in later follow-up periods. The study highlights gender disparities, with boys experiencing more notable declines in healthcare consultations, especially in specialist care. Additionally, the time trends varied across different diagnostic groups. While consultations for sleep problems and anxiety/depression were consistently below the predicted levels, declines in ADHD occurred later in the pandemic. Consultations for ASD were in line with predictions much of the time. However, there were periods with lower-than-predicted consultations before, during, and after social restriction phases. The data suggests an overall decline in healthcare utilization rather than a shift between sectors. Conclusion The findings offer insights into healthcare utilization during pandemic times among children and adolescents with pre-existing mental health diagnoses. The study underscores the importance of continuous monitoring and support for this group, ensuring accessible and responsive healthcare during public health emergencies.