The COVID-19 pandemic posed an unprecedented global challenge, with past evidence suggesting negative psychological effects with the additional concern that social and physical restrictions might disproportionately affect adolescents.

To explore mental health and its wider determinants in young people in the UK during 1 year of the COVID-19 pandemic (August 2020–August 2021).

A representative sample of 11 898 participants (48.7% female) aged between 13 and 19 years (mean = 16.1) participated in five waves of data collection. Using validated self-reported questionnaires for loneliness, anxiety and depression, this survey measured the extent and nature of the mental health impacts of the coronavirus pandemic and help-seeking behaviours, and changes over time.

Young people experienced higher levels of anxiety during the summer and fall 2020, followed by higher levels of depression during the winter 2020–2021, with loneliness gradually increasing then peaking during the spring and summer of 2021. Young people who were older, female, with pre-existing mental-health issues and experiencing financial difficulties were at higher risk of anxiety, depression and loneliness. Help-seeking behaviours reduced the risk of depression and loneliness.

The COVID-19 pandemic had substantial impact on young people, whether on their mental health, their social contacts and interactions or their perspective on what the future holds for them. Young people strongly advocated for better teacher training, and a better integration of mental health services, particularly within their schools.