ABSTRACT UK young adults saw sharp mental health declines during the COVID-19 pandemic. This paper examines whether living with siblings helped moderate this negative effect. We compare the outcomes of young adults (age 19), i) who were living with parents and siblings, with ii) those who were living with only parents, and iii) with those who were living away from parents. We used data from the Millennium Cohort Study COVID-19 survey, linked with the mainstage survey (N = 2,578), and captured mental health with: the Shortened Warwick-Edinburgh Mental Well-being Scale and the Kessler-6 Psychological Distress scale. As young men and women may be differently affected by sibling co-residence, we vary living arrangements effects by gender. While average young adult mental health deteriorated during the first national lockdown, there were variations by gender and living arrangements. For young men, living with siblings was associated with improved mental health on both measures during the first COVID-19 lockdown. For young women, living with parents was associated with lower psychological distress than living away from home, but siblings provided no additional benefit. Data from later in the pandemic suggests that, as young adults became more accustomed to social restrictions, the importance of family living arrangements declined.