A strong emphasis has been placed on social-emotional learning (SEL) during the preschool years; however, there is a dearth of research into the drivers of program success in this setting. This study examined the effectiveness of a formal, teacher-led social-emotional learning program, COPE-Resilience, on the development of 4 to 5-year-old children in an Australian preschool setting. Ninety-one preschool children from three classrooms in an early learning centre in Melbourne, Australia were allocated to receive the six-week COPE-Resilience intervention by experienced COPE-Resilience facilitators (n = 33), first-time program facilitators (n = 29), or participate as a non-intervention comparison group (n=29). Results indicated that children undertaking COPE-Resilience with an experienced teacher facilitator demonstrated the greatest improvements in teacher-rated empathy, prosocial behaviours, coping styles, inhibitory control, and problem behaviours. These results indicate that the level of program facilitation experience enhances implementation success. Consideration for the impact and implications of program delivery experience in evaluation design are discussed.