This study examined the effectiveness of the attachment-based Circle of Security 20-week intervention (COS) in improving parent emotional functioning in 83 families referred to a community clinical service with concerns about their young children’s behavior. Parenting stress and parent psychological symptoms were assessed pre and post intervention and mixed design repeated measures ANOVAs were used to assess change. Severity of presenting problems was considered as a moderator. Results showed clinically significant improvements in both aspects of parent emotional functioning, with changes accounted for by for those with more severe problems at the outset. Improvements for parents were associated with improved child behavior and more positive parent representations of the child and of parenting capacity. Findings suggest the intensive COS intervention is effective in reducing parenting stress and psychological symptoms in parents of children from aged 1–7 years. Questions remain about the mechanisms of change and the direction of effects.