This study examines the intensity effect of a mentoring intervention for Head Start teachers, the Individualised Learning Intervention (ILI), as it impacts child social emotional outcomes. Pairs of Mentor and Protege teachers across three sites in two states were randomly assigned to intervention and control groups. Mentors in the intervention group participated in a 50-hour mentor seminar and supported the intervention Protege teachers' professional growth and development across one Head Start school year. Mentors themselves were also supported by a Mentor Coordinator. As part of a quasi-experimental study of implementation fidelity effects, the intensity of mentoring intervention was measured by the Intensity of Intervention Scale and the intervention group was divided into high and moderate intensity levels. Evidence is presented for the effects of high intensity levels of the ILI Mentoring programme, relative to moderate intensity and control conditions, on the children in the Protege teachers' classrooms. These results lead to practice and policy implications regarding the implementation of mentoring programmes such as the ILI which are designed for early childhood educators and built upon adult self-directed learning experiences and collaborative support. (author abstract)