Past meta-analyses show that both child–mother and child–father attachment insecurity are independently and jointly associated with more externalizing behaviors in children. Little is known, however, on the ways that different types of insecure attachment independently and jointly predict the development of externalizing behaviors over time. Existing work also neglects the impact of children’s gender within the context of child–father relationships. The current study addresses these limitations by investigating how insecure type of child–father attachment, child–mother attachment, and their interaction in the preschool years predict boys’ and girls’ externalizing behaviors in middle childhood, when controlling for children’s externalizing behaviors in the preschool years. The sample included 144 preschool-aged children (M = 46.89 months, SD = 8.77, 83 girls) and both of their parents. At Time 1, children completed independent separation-reunion procedures with each parent, which were coded using the Preschool Attachment Rating Scales. At Time 1 and Time 2 (5 years later), mothers and fathers completed the Strengths and Difficulties Questionnaire to report on their children’s externalizing behaviors. Results showed no systematic differences in the way that child–mother and child–father attachment predicted the development of externalizing behaviors in boys and girls. Across all children, results identified an interaction of child–father and child–mother ambivalence, by which the presence of ambivalence toward both parents predicted the development of more externalizing behaviors. In addition, child–father controlling-caregiving attachment predicted the development of fewer externalizing behaviors. These results provide insight into the ways that insecure child–father and child–mother attachment predict later socioemotional adaptation.