Self-reported data and media suggest youth mental health is declining. To more objectively measure this, we conducted a population-wide investigation of changes in diagnosed point prevalences of anxiety/depression among children and young adults between 1997 and 2017 in Ontario, Canada. All Ontarians (population approximately 14 million) age 5-25 in each index year were included and grouped as follows: 5-10, 11-15, 16-20, 21-25. As the Canadian medical system is public and universal, all diagnoses can be tracked via billing submissions. The outcome of interest was non-psychotic anxiety/depression diagnosed by any physician in any setting. Using regression analyses, cross-sectional administrative (billing) data for 1997, 2002, 2007, 2012 and 2017 for diagnoses of anxiety/depression were linked to indicators of sex, household income, rurality, and immigrant/refugee status. Point prevalence of anxiety/depression diagnoses increased with age (girls 2·1 to 16·9%, boys 2·9 to 10·6%), particularly from age 11 to 20. Rates, overall, remained stable until 2012 then rose among 11-15 and 16-20 year-olds. This pattern varied by sex. An earlier inverse association with rural residency disappeared by 2017, while immigrant status aligned increasingly with absence of diagnosed anxiety/depression. Lowest household income quintile was associated with higher prevalence of diagnoses. Cross-sectional data preclude hypothesising about causes of observed shifts. We found no overall upsurge in youth anxiety/depression, although small increments between 2012 and 2017, and variability with sociodemographic characteristics suggest a possible looming trend and the merit of studying concomitant and potential explanatory shifts in social circumstances. Copyright © 2021. Published by Elsevier B.V.