Previous studies have demonstrated deteriorations in young adult mental health during the COVID-19 pandemic, but evidence suggests heterogeneity in the mental health impacts of the pandemic. We sought to identify factors which may predict changes in psychological distress and wellbeing during the COVID-19 pandemic in UK young adults. A total of 2607 young adults from the Millennium Cohort Study were included. Psychological distress and mental wellbeing were measured using the Kessler-6 and Short Warwick-Edinburgh Mental Wellbeing Scale, respectively. Assessment occurred at three timepoints between the ages of 17-19: 2018/19 (pre-COVID Baseline), May 2020 (COVID Wave 1) and September/October 2020 (COVID Wave 2). Latent change score models were used to study change in distress and wellbeing across the study period, as well as the impact of sex, relative family poverty, parental education, preexisting mental health difficulties and perceived social support on these changes. The latent change score models suggested both distress and wellbeing tended to increase across the study period. Being female and in relative poverty predicted greater increases in distress and/or poorer wellbeing. Higher levels of parental education and greater perceived social support were protective against increased distress and associated with improved wellbeing. The impact of the COVID-19 pandemic on UK young adult mental health is complex. We provide further evidence for a distinction between symptoms of poor mental health and wellbeing. Research is urgently needed to assess the long-term impacts of the COVID-19 pandemic on the mental health and wellbeing of young people, particularly in more vulnerable groups.