The development of mastery behavior is important because motivation is essential for learning. Previous research has concentrated on motivation in school-age children, but less work has been done on understanding the development of mastery behavior in young children. This study examined the effect of maternal affect (facial and verbal tone) and sensitivity (supportive presence, quality of assistance, and support of development) during a teaching task on toddler persistence during independent task engagement. Toddlers (20 months old) and their parents (N=92) participated in a teaching task in which each parent taught the toddler how to use a difficult toy, then the child engaged with the toy independently. Although, neither aspect of maternal affect during teaching was significantly related to persistence, two aspects of maternal sensitivity—maternal quality of assistance and maternal support of development—were found to be significant predictors of child persistence during independent task engagement. Findings suggest that providing the appropriate level of assistance and keeping toddlers focused on the task goals were more predictive of toddler mastery motivation than a positive disposition or a providing a supportive atmosphere (i.e., secure base).