We used latent profile analysis on a longitudinal dataset to examine changes in maternal and child mental health during COVID-19 and factors that may protect against declines in mental health. Participants were 183 low-income mothers (M = 36 years) with young children (M = 5.31 years) in the City of Toronto with data collected prior to and during the pandemic in 2020. Mothers reported on their own stress, anxiety and depression and their children's emotional, conduct, hyperactivity, peer, and prosocial problems at both timepoints. We found heterogeneity in mental health changes, with 5 distinct patterns of change for mothers, and 4 distinct patterns of change for children during COVID-19. The majority (83%) of mothers experienced significant declines in at least one aspect of mental health. In contrast, the majority of children (65%) experienced either no change or improvements in mental health. Interestingly, patterns of change across these groups were not differentiated by demographic characteristics such as income, education, and family composition. However, for mothers, a higher degree of satisfaction with social support was associated with membership in a profile with better mental health both prior to, and during the pandemic. For children, having a stable history of early childhood education, and care was associated with membership in a profile that showed improvements in mental health during the pandemic. We discuss how our results support the need for proactive and global interventions for at-risk families with raised mental health concerns, and the benefits that stable early childhood education and care may provide for young children.