Given that maternal support promotes healthy social and emotional development in early childhood, it is important to understand the predictors of such support, especially during emotional challenges. In this study, mothers’ dispositional distress reactivity (i.e., the tendency toward experiencing distress in response to children’s negative emotions and behavior) was assessed as a predictor of maternal support in a given moment when children showed within-person fluctuations in aversive behavior (i.e., negative affect and disruptive behaviors) in concurrent and prior moments. Data were collected when children were 33 months of age. Mothers (N = 128) reported on their distress reactivity, and maternal support and child aversive behavior were coded in 15-s intervals during a 5-min snack-delay task. As hypothesized, multilevel models revealed that mothers’ dispositional distress reactivity predicted decreases in maternal support when children showed within-person increases in aversive behavior in the prior 15-s interval but not in the concurrent interval. Findings highlight the importance of investigating the contributions of maternal dispositional tendencies to moment-to-moment changes in parenting behavior during moderate, everyday challenges with young children.