Recognition of the protective value of secure parent–child relationships has prompted a growing interest in parenting interventions informed by attachment theory. Circle of Security Parenting (COS-P) is one such program, specifically designed for scalability. Although widely disseminated internationally, evidence for the effectiveness of COS-P is very limited. This non-randomized controlled effectiveness study was designed to help address this gap. A sample of 256 parents of children aged 0–6 years was recruited from four community child and family health organizations. Assessments were undertaken pre- and post-intervention for the treatment group (n = 201) and at comparable times for the waitlist control group (n = 55). Analysis of data for mothers (89% of sample) revealed a significant Time x Group interaction for six of the seven outcomes examined. Compared to mothers in the control condition, treatment group mothers reported significantly: (a) improved parental mentalizing and self-efficacy regarding empathy and affection toward the child; (b) reduced caregiving helplessness and hostility toward the child; and (c) reduced depression symptoms, at the end of COS-P treatment. There was no difference between groups for change in perceived child difficultness. Within-treatment-group analyses indicated that mothers with older children reported greatest reductions in caregiving helplessness, and mothers with probable clinical depression pre-intervention reported greatest reductions in hostility and depression symptoms. Improvements in other study outcomes did not differ by depression severity or child age. Exploratory analyses indicated that treatment group fathers showed the same pattern of change as mothers. Clinical implications and directions for future research are discussed.