This study aims to explore school administrators' insights into the key competencies that play an important role in achieving success in English language teaching and learning at government primary schools in Thailand. The study employed semi-structured interviews with six school administrators, including four principals and two head teachers of foreign language departments from Bangkok and five regions in Thailand. Based on SEAMEO INNOTECH's (2015) "Success Competencies of Southeast Asian School Heads," the results reveal that school administrators' essential competencies include strategic thinking and innovation, managerial leadership, instructional leadership, personal excellence, and stakeholder engagement. These competencies are overlapped, interconnected, and underpinned by complex factors including school administrators' visions and administrative policies, English as a medium of instruction, Thai teachers, foreign teachers, teacher professional development, students' learning behaviors, learning materials and ICT, English language development activities, family engagement, and the strong support of the Ministry of Education. These factors are not mutually exclusive; they are, rather, interwoven in the sophisticated educational networks that require consistent and mutual collaborations from all relevant stakeholders who will gear the students towards English language learning success. School administrators' competencies and leadership skills should be enhanced for effective administration. Professional learning community partnerships need to be initiated and strengthened for long-term collaborations in order to enhance sustainable development of English language teaching and learning in primary education in Thailand.