Objectives The COVID-19 pandemic has been associated with increased mental health problems. We investigated (1) associations between disordered eating in adolescence and mental health problems after one year of the pandemic and (2) the mechanisms explaining associations. Method We analyzed data from a population-based birth cohort in Quebec, Canada (557 males and 759 females). High and low levels of disordered eating symptom trajectories were previously estimated (age 12, 15, 17, and 20 years). Anxiety, depression, non-suicidal self-injury, and suicidal ideation were assessed at 23 years (March–June 2021). Putative mediators included loneliness and social media use (age 22 years, July–August 2020). Analyses controlled for mental health and socio-economic status at age 10–12 years and were conducted for males and females separately. Results Females in the high-level disordered eating symptom trajectory were at increased risk for non-suicidal self-injury (OR 1.60; 95% CI 1.02–2.52) and suicidal ideation (2.16; 1.31–3.57), whereas males were at increased risk for severe anxiety (2.49; CI 1.11–5.58). Males and females in the high-level trajectory were more likely to report severe depression (2.26; 1.14–5.92 and 2.15, 1.36–3.38 respectively). Among females, associations were partially explained (17–35%) by loneliness during the first 4 months of the pandemic. Conclusion Young adults who experienced disordered eating as adolescents were at increased risk of mental health problems during the pandemic. Loneliness partially mediated the effect, suggesting that pandemic mitigation resulting in increased social isolation may have exacerbated mental health problems among women with a history of disordered eating. Supplementary Information The online version contains supplementary material available at 10.17269/s41997-022-00715-8.