There is evidence of an overall decline in women's mental health, particularly those with young children, in the wake of the COVID-19 pandemic. However, research has also found heterogeneity in women's mental health responses. This longitudinal study sampled low-income women with young children by recruiting from the government's child care financial subsidy waitlist. To examine heterogeneity in women's mental health responses to COVID-19, a latent transition analysis was employed to identify profiles of anxiety, depression, and stress among 289 low-income mothers. Using these identified profiles, we examined the transitional patterns between profiles before and during COVID-19 and the sociodemographic and familial factors related to these profiles. A three-profile solution was identified prior to COVID-19 and a four-profile solution during COVID-19, with some profiles exhibiting qualitatively different defining characteristics. Latent transition analyses found diverse patterns of mental health changes after the onset of COVID-19. Mothers with better mental health prior to COVID-19 tended to have the most stable mental health during COVID-19. In contrast, mothers who were highly stressed prior to COVID-19 were equally likely to improve or decline after the onset of the pandemic. In addition, the relationships between race, parenting practices, child temperament, and child mental health were significantly related to mothers' mental health profiles. These findings describe mothers' experiences and areas where policymakers and practitioners can tailor support to low-income women with young children.