This study tested the association between parent illicit drug use disorder (DUD) in early adulthood and observed parenting practices at ages 27-28 and examined the following 3 theoretically derived models explaining this link: (a) a disrupted parent adult functioning model,(b) a preexisting parent personality factor model, and (c) a disrupted adolescent family process model. Associations between study variables and child externalizing problems also were examined. Longitudinal data linking 2 generations were drawn from the Seattle Social Development Project (SSDP) and The SSDP Intergenerational Project (TIP), and included 167 parents and their 2- to 8-year-old child. Path modeling revealed that parent DUD in early adulthood predicted later observed low-skilled parenting, which was related to child externalizing problems. The preexisting parent personality factor model was supported. Parent negative emotionality accounted for the association between parent early adult DUD and later parenting practices. Parent negative emotionality also was related directly to child externalizing behavior. Limited support for the disrupted transition to adulthood model was found. The disrupted adolescent family process model was not supported. Results suggest that problem drug use that occurs early in adulthood may affect later parenting skills, independent of subsequent parent drug use. Findings highlight the importance of parent negative emotionality in influencing his or her own problem behavior, interactions with his or her child, and his or her child's problem behavior. Prevention and treatment programs targeting young adult substance use, poor parenting practices, and child behavior problems should address parent personality factors that may contribute to these behaviors.