The aim of the present study is to examine whether the COVID-19 pandemic has increased the risk of mental health problems (MHP) in adolescents nine months post-outbreak. For this purpose, a longitudinal cohort study was conducted based on a probability sample of the Dutch population. We compared the prevalence and incidence of MHP in 16–20 year-old adolescents in November-December 2020 (N = 251) with the prevalence and incidence in adolescents in November-December 2012 (N = 346) and November-December 2016 (N = 253). Results showed a higher prevalence of moderate anxiety and depression symptoms in the 2020 than in the 2012 and 2016 cohorts, but differences in mean scores were absent or small. The prevalence of sleep problems, fatigue, use of medicines for symptoms did not differ between the three cohorts. The use of mental health services was more prevalent in the 2020 than in 2016 cohort, but there was already a statistical trend of higher use in the 2016 compared to the 2012 cohort. No differences in the incidence of any MHP, based on data of the previous year (2011, 2015, and 2019, respectively) were found. Results suggest a very limited negative effect of this pandemic on MHP among Dutch adolescents 9 months post-COVID-19 outbreak.