The purpose of this study was to evaluate the effectiveness of a 2-component intervention for biological and foster parent (pairs) to improve parenting practices, co-parenting, and child externalizing problems. Participants were biological and foster parents (N = 128) of primarily neglected children (ages 3 to 10 years) placed in regular foster homes. Biological and foster parents were randomly assigned in pairs to the intervention (n = 80) or a usual care (n = 48) condition. Intervention families received a 12-week parenting course (Incredible Years) and a newly developed co-parenting component. Key findings included significant gains in positive parenting and collaborative co-parenting for both biological and foster parents at the end of the intervention. At follow-up, intervention parents sustained greater improvement in positive parenting, showed gains in clear expectations, and reported a trend for fewer child externalizing problems. Findings supported the feasibility of offering joint parenting training to meet the needs of participating families and demonstrated that the co-parenting construct applied to families in the foster care system was amenable to intervention.