OBJECTIVES: Findings from a birth cohort study indicated that the mental health of young adults had not worsened during the first wave of the COVID-19 pandemic, compared to 2018. This study examined longitudinal changes in mental health between March 2018 and June 2021 in the context of protracted public health mitigation measures about 12 months after the onset of the pandemic. METHODS: Participants from the Quebec Longitudinal Study of Child Development (n = 2120 at inception; n = 1461 during the COVID-19 pandemic), a population-based cohort of individuals born in 1997/1998, reported on their depressive and anxiety symptoms as well as suicidal ideation prior to the pandemic in 2018 (age 20), and during the pandemic in the summer of 2020 (age 22) and spring of 2021 (age 23). RESULTS: Depressive (Cohen's d = 0.15 [95% CI: 0.09 to 0.20]) and anxiety (Cohen's d = 0.33 [95% CI: 0.27 to 0.39]) symptoms increased between 2018 and 2021 for both males and females, but suicidal ideation did not change. There was also a significant increase in moderate to severe depressive (31.7% to 36.3%) and anxiety (14.7% to 24.8%) symptoms from 2018 to 2021. Youth who were students, those who were experiencing financial stress, food insecurity, and loneliness, and those without pre-existing poor mental health experienced the largest increase in depressive and anxiety symptoms over time. CONCLUSION: These findings highlight the mental health burden experienced by young adults during the COVID-19 pandemic, highlighting the need for preventive services and continued longitudinal follow-ups of these youths.