Background: The COVID-19 pandemic has had a substantial impact on children and families worldwide. Children's mental health has been at the forefront of pandemic research, with several observational studies documenting its decline. Limited person-centred research exists, however, investigating the diverse mental health responses of vulnerable children during COVID-19. Objective: The purpose of this study is to examine the profiles of 289 low-income children's mental health transitions from pre-COVID-19 to during COVID-19. Methods: Mothers' reports of children's mental health using the Strengths and Difficulties Questionnaire from before ([x-bar] = 2.73 years, SD = 0.23) and during ([x-bar] = 5.31 years, SD = 0.59) COVID-19 were used. Results: Three comparable profile solutions were identified pre- and early during the pandemic. Latent transition analysis revealed diverse patterns of children's mental health trajectory from prior to during COVID-19. Based on transition probabilities, the majority of children in the "Average Levels of Internalizing/Externalizing Problems" and "Externalizing Problems" profiles pre-pandemic showed stability in profile membership. Interestingly, most children in the high levels of "Internalizing/Externalizing Problems" profile pre-pandemic experienced some improvement in their mental health. Pre-pandemic maternal mental health and parenting had significant associations with children's profile membership at both time points. Conclusions: Our findings reveal the heterogeneity in children's mental health responses in times of large-scale crises. They also identify how pre-existing maternal risk factors may underlie the diverse experiences of children who underwent declining, stable, or improving mental health profiles during the COVID-19 pandemic.