ObjectivesAdolescents and young adults have been greatly affected by quarantine measures during the COVID-19 pandemic, but little is understood about how restrictions have affected their well-being, mental health, and social life. We therefore aimed to learn more about how UK quarantine measures affected the social lives, mental health and well-being of adolescents and young adults.DesignQualitative interview study. The data were analysed using reflexive thematic analysis, with particular attention paid to contextual factors (such as age, gender, ethnicity and health status) when analysing each individual transcript.SettingData collection took place remotely across the UK via audio or video call, between June 2020 and January 2021.ParticipantsWe conducted semi-structured interviews with 37 participants (aged 13–24 years) to elicit their views.ResultsAuthors generated four themes during the qualitative analysis: (a) concerns about disruption to education, (b) missing social contact during lockdown, (c) changes to social relationships and (d) improved well-being during lockdown. Many participants said they struggled with a decline in mental health during the pandemic, lack of support and concern about socialising after the pandemic. However, some participants described experiences and changes brought on by the pandemic as helpful, including an increased awareness of mental health and feeling more at ease when talking about it, as well as stronger relationship ties with family members.ConclusionsFindings suggest that young people may have felt more comfortable when talking about their mental health compared with prepandemic, in part facilitated by initiatives through schools, universities and employers. However, many were worried about how the pandemic has affected their education and social connections, and support for young people should be tailored accordingly around some of these concerns.