Dyadic interactions associated with maternal depression and anxiety may perpetuate maternal mental illness and impact infant attachment. Individual and maternal-dyadic therapies are effective but resource intensive. We assessed feasibility, acceptability, and preliminary efficacy of a newly developed maternal-infant dyadic group therapy intervention. This was an open-label pilot study targeting mothers with mood or anxiety disorders, and their infants aged 6 to 12 months. We conducted three 12-week groups combining evidence-based maternal and mother-infant dyadic strategies to enhance mood, insight, parenting, and mentalizing capacity. We measured recruitment and retention rates, reasons for nonparticipation, and missed sessions. Acceptability of the intervention was assessed via questionnaires and semi-structured interviews. Efficacy outcomes were the Parenting Stress Index (PSI), Edinburgh Postnatal Depression Scale (EDPS), and the Beck Anxiety Inventory (BAI), measured pretreatment and posttreatment. The feasibility and acceptability were excellent. There was a significant reduction in mean depressive symptom scores (t 3.31; p 0.008 sig) and a trend toward decreasing anxiety scores (t 1.96; p 0.08). The total PSI score decreased, approaching statistical significance (t 2.23; p 0.057). Enhanced insight, parenting capacity, affect regulation, and positive interaction with baby were supported with self-report surveys and interviews. This resource-efficient novel mother-baby dyadic group intervention shows excellent feasibility, acceptability, and has good preliminary efficacy results. It has the potential to improve depression, anxiety, affect regulation, parenting, and maternal mentalization.