In the face of COVID-19, many schools have to educate their students using online activities. During this time, whether and how parents are involved may be of particular importance for young children—who are less able to learn independently via the Internet due to their developmental immaturity. Therefore, this study examined the cross-sectional association of maternal involvement in child online learning with child adjustment during the COVID-19 pandemic and tested maternal mindfulness as a moderator. Data were collected from 236 mothers of kindergarten-aged children (mean age = 55.91 months; 75% of them were girls) during the fourth wave of COVID-19 outbreak in Hong Kong, China. Using paper-and-pencil questionnaires, mothers rated their involvement and mindfulness and their children’s pre-academic ability and internalizing and externalizing behaviors and provide demographic information. Regression models revealed that maternal involvement was associated positively with child pre-academic ability and negatively with child internalizing behaviors, but such associations were only significant for children with more mindful mothers. Maternal mindfulness did not moderate the negative association between maternal involvement and child externalizing behaviors. Findings highlighted the role of maternal mindfulness in child development, suggesting that it may be crucial to promote maternal involvement and mindfulness during the pandemic and perhaps beyond.