Socioeconomic disparities in mental health have been reported during the COVID-19 pandemic. However, few studies have examined the mechanisms through which such disparities in mental health occurred. This pre-registered study aimed to examine socioeconomic disparities, as indexed by education levels, in the trajectory of mental health at the early stages of the COVID-19 pandemic and whether financial stress associated with the pandemic mediated socioeconomic disparities in mental health. Data were drawn from the Love in the Time of COVID project, of which we included four waves of data (N = 2,204) collected between March 27th and June 21st, 2020. Education was assessed at baseline, and mental health outcomes (i.e., eudaimonic well-being, positive affect, negative affect, depressive and anxious symptoms) and financial stress associated with the COVID-19 pandemic were assessed at each wave. Results indicated that there were educational disparities in eudaimonic well-being, negative affect, and depressive and anxiety symptoms at baseline, with those with lower education levels reporting poorer mental health. However, education did not amplify disparities in mental health outcomes over time, showing no associations with the rates of change in mental health outcomes. Financial stress mediated the associations between education and eudaimonic well-being, negative affect, and depressive and anxious symptoms at baseline, and there were no temporal variations in the mediation effects of financial stress. These results highlight persistent educational disparities in mental health at the early stages of the COVID-19 pandemic, and such educational disparities may be partially explained by financial stress associated with the COVID-19 pandemic.