Because the COVID-19 pandemic has been implicated in increased mental health concerns for families of low income, we aimed to describe maternal perspectives about the pandemic's impact on their kindergartener's mental health during the 2020-2021 school year. We conducted 22 in-depth, semi-structured interviews with U.S. mothers with low income who had kindergarten-age children (50% male and 50% female). All participants were female, ranging in age from 24 to 44 years, and reported the following ethnic/racial identities: non-Hispanic Black or African American (41%), Hispanic of any race (36%) and non-Hispanic, White (23%). With a team comprising multiple researchers from varied disciplines (e.g., medicine, education and public health) our analytic process used an iterative approach for developing and revising codes and themes until we reached thematic saturation. Most mothers described negative impacts on social, behavioural and emotional aspects of mental health. Some described positive social impacts, including strengthened family relationships. Mothers described no positive changes to behavioural or emotional aspects of mental health. Maternal perspectives suggest the importance of prioritising access to screening and treating mental health needs to support children's kindergarten transition, mitigate pandemic impacts and plan for future disruptions.