Young children’s social–emotional development is powerfully shaped by their early environments, which for many includes early childhood education (ECE). Infant and Early Childhood Mental Health Consultation (IECMHC) pairs teachers and infant and early childhood mental health (IECMH) consultants to promote teachers’ capacity to foster positive social–emotional development in ECE. Although the outcomes of IECMHC have been well studied, little research has investigated how this model leads to changes for teachers and children. According to theory, the quality of the relationship between teachers and IECMH consultants, termed consultative alliance (CA), is a key mechanism of change. This study analyzed the role of CA on 6-month outcomes of IECMHC in a sample of 316 children, 289 teachers, and 62 IECMH consultants. Results from multilevel models suggested that stronger CA predicted greater improvements in teacher– child closeness and teacher-rated child attachment behaviors. In addition, a strong CA was related to greater improvement in classroom climate, teachers’ self-efficacy, and teachers’ perceptions of their jobs. This study upholds the centrality of relationship-building and parallel process in mental health consultation, and by advancing understanding of the mechanisms of change for IECMHC may provide salient implications for policy and practice. (author abstract)