The rise in mental health problems among adolescents in high-income countries presents a challenge to service systems. For the development of services, there is a need for better insight into temporal psychiatric treatment-trends and outcomes. This study aims to analyze time-trends in both psychiatric treatment patterns and outcomes, utilizing a national sample of all adolescents receiving psychiatric treatment in Finland from 2003 to 2013. For time-trend-analysis, the sample was divided into two cohorts, using the onset year of 2008 as a cutoff. For each case, information on psychiatric treatment was gathered from registers within a five-year follow-up period from the onset of treatment or to death. The association between the inclusion year and outcome variables was studied via weighted generalized linear models. Adolescents in the latter cohort had a greater proportion (p <
0.001) of mood and anxiety diagnoses, a lower likelihood of hospitalization, a higher average of outpatient visits, and greater usage of psychotropics (excluding benzodiazepines). Those whose treatment began after 2008 were more likely to be alive (baseline characteristic adjusted Odds Ratio (aOR): 0.7, 95%CI: 0.6-0.8) and still in treatment contact (aOR: 1.4, 95%CI: 1.3-1.4) after four years from the onset. There was no difference in the long-term disability ratio. The results indicate favorable developments towards outpatient care in mental health services for adolescents with a significant decrease in mortality. Approaches to further developing cost-effective, personalized mental health services are discussed.