Early childhood development (ECD) is instrumental to shaping educational, emotional, and economic trajectories; alleviating poverty; and achieving gender equality. Pediatricians are experts in children's health and trusted sources of guidance for families and clinicians and thus are optimal ECD champions. This case study describes collaboration by the American Academy of Pediatrics, Kenya Pediatric Association, and Pediatric Association of Tanzania to activate pediatricians as ECD champions in Kenya and Tanzania. From July 2020 through January 2021, the collaborators assessed ECD needs by interviewing 20 key informants per country from governmental ministries, nongovernmental organizations, and clinical practice and assessing datasets and policy documents. In 2021, the societies recruited 15 pediatricians per country as champions; surveyed their knowledge, attitudes, and practices; and trained them on 4 core competencies: understanding early brain development science; developmental and behavioral screening, surveillance, and diagnosis; integration of ECD promotion into clinical practice; and advocacy skills for ECD and nurturing care. In 2021, each society established advocacy-in-action projects to advance ECD. In Kenya, the cohort surveyed clinicians on barriers to ECD, implemented a 2-day in-person training for 90 providers, and developed a 5-week Fundamentals of ECD course, taken by 113 pediatricians from 7 African countries. In Tanzania, champions conducted ECD training workshops for 78 health managers and 189 health care providers in 9 facilities in 7 regions and established 9 ECD corners with toys and information in health care facilities. These results highlight considerations for supporting ECD, including building on existing strengths, infrastructure, and networks; strengthening ECD knowledge among pediatricians; and advocacy skillbuilding.