Using data from the Fragile Families and Child Wellbeing study (n = 1553), the present study examined whether father involvement and co-parenting quality mediated the association between union instability (number of residential romantic partner changes) over the first 5 years of life and children's externalizing problem behaviors (EPB) at 5 years. The results indicate that only co-parenting support mediated the association between union instability and children's EPB, controlling for known covariates of children's EPB. The findings suggest that the union instability associated with romantic partner transitions has a deleterious effect on children's behaviors because the change in union status decreases the quality of the co-parenting relationship.