This article uses data from the Fragile Families and Child Wellbeing Study to examine whether family instability is associated with changes in perceived social support, material hardship, maternal depression, and parenting stress among mothers of young children. In addition to accounting for the number of transitions that a mother experiences during the first five years of her child's life, we pay close attention to the type and timing of these transitions. We find that mothers who transition to cohabitation or marriage with their child's biological father experience declines in material hardship and that those who transition to cohabitation or marriage with another man exhibit modest declines in both material hardship and depression. Mothers who exit cohabiting or marital relationships encounter decreases in perceived social support and increases in material hardship, depression, and parenting stress. Overall, our results suggest that both the type and, to a much lesser degree, the timing of family structure transitions may influence maternal well-being.