This study used a longitudinal probability sample survey, "Understanding Society: COVID-19," to examine trajectories in adolescents' mental health, via the Strengths and Difficulties Questionnaire, at three timepoints during COVID-19 with a particular emphasis on vulnerable groups (i.e., young carers, adolescent girls, BAME--Black, Asian and minority ethnic, and adolescents in financially strained households). Generally, self-reports of emotional and total difficulties remained stable during the pandemic, although adolescents who had limited social support were far more likely to report severe emotional and total difficulties. Young people with pre-existing mental and physical health conditions appeared more resilient, whereas vulnerable young people were hit the hardest during the pandemic. Compared to their less vulnerable peers, young carers, adolescent girls, Black or mixed-race young people and adolescents in financially strained households were more likely to report reduced mental health during the pandemic. It is hoped that the findings will contribute to debates about the pandemic unveiling existing mental health inequalities in society, and to public policy in an era of perma-crises as we currently face a cost of living crisis where public services are under enormous strain to reach those who need them most.