BackgroundHappiness may help to prevent negative physiological outcomes in response to life events; however, factors contributing to happiness during the COVID-19 pandemic have not been longitudinally investigated. This study explored the predictors of happiness in mothers of young children in Japan using comparable data that were obtained before and during the COVID-19 pandemic.MethodsWe conducted the baseline survey in February 2020, and 4 months later, we also conducted the follow-up survey. Throughout all 47 prefectures in Japan, 4,700 (100 respondents/prefecture) mothers of infants and/or preschoolers (0–6 years) participated in the baseline online survey; 2,489 of these also participated in the follow-up survey.ResultsWe performed hierarchical multiple regression analysis and our final model indicated that maternal happiness during COVID-19 pandemic was positively related to employment status (homemaker, β = 0.052, p = 0.014), levels of available social support (average, β = 0.052, p = 0.012, high, β = 0.055, p = 0.010) and happiness score before the pandemic (β = 0.467, p < 0.001), and satisfaction toward the measures against the COVID-19 at partners’ workplace (average, β = 0.129, p < 0.001; high, β = 0.279, p < 0.001), preventive behavior against COVID-19 (average, β = 0.055, p = 0.002; high, β = 0.045, p = 0.015) and positive attitudes/thinking (β = 0.087, p < 0.001) during the pandemic. In contrast, poor mental health (K6 ≥5, β = −0.042, p = 0.011) before the pandemic and negative changes during the pandemic (≥3, β = −0.085, p < 0.001) were negatively related to maternal happiness during the pandemic. Our final model explained 44.9% of the variance in mothers’ happiness during the COVID-19 pandemic.ConclusionsSatisfaction toward the measures against the COVID-19 at partners’ workplace, preventive behavior, and positive attitudes/thinking were especially important for maternal happiness during the COVID-19 pandemic. Future study is needed to consider measures against infectious diseases in the workplace that are desirable for the well-being of parents with young children, taking into account the gender perspective.Supplementary informationThe online version contains supplementary material available at https://doi.org/10.1265/ehpm.22-00008.