<p>The mother–social worker working alliance is critical to the success of mental health interventions delivered within the child welfare system. Maternal characteristics may influence alliance development, yet few studies have examined these dynamics from both mother and social workers’ perspectives. Guided by ecological theory, this study explores how micro-level factors (e.g., demographics, socioeconomic status, trauma history) and macro-level factors (e.g., prior involvement with the foster care or family court systems) shape the quality of the working alliance between mothers and social workers participating in the Parenting-STAIR (P-STAIR) program, a trauma-focused parenting intervention for mothers and children involved in child welfare. Data were drawn from 112 mothers enrolled in an open trial of P-STAIR. Participants were referred by four preventive service agencies and met criteria for PTSD. Working alliance was assessed separately by mothers and social workers using the WAI-S. Multiple linear regression analyses examined associations between micro- and macro-level factors and working alliance outcomes. Sixty-one mother–social worker dyads contributed to the working alliance assessment. Mothers who speak Spanish reported higher working alliance scores (<i>b</i> = 3.67, <i>p</i> &lt; .05), while social workers rated mothers with prior foster care involvement lower on alliance (<i>b</i> = − 8.94, <i>p</i> &lt; .01). Trauma history was not a significant predictor of working alliance. Micro- and macro-level factors differentially shape the mother–social worker alliance. Clinicians should consider potential language barriers and clients’ prior system involvement when fostering engagement in child welfare–related interventions.</p>