We investigate whether mother’s poor union quality attenuates the association between union instability and young children’s behavioral adjustment. Using data from three waves of the Fragile Families and Child Well-being Study to consider children born to married or cohabiting mothers (N = 1730), we determine that children who have experienced poor union quality between mothers and their partners have higher predicted aggressive behavior scores at age 3, regardless of whether they have experienced union transitions, compared to children who have experienced high-quality, stable unions. Children who have experienced instability in the context of higher-quality unions and relatively less acrimonious dissolutions are similar to children raised in high-quality stable unions in terms of predicted aggressive behavior scores.