High-quality population data on social and emotional development, from infancy to young adulthood is essential for evidence based investment in the health and wellbeing of young populations. While population monitoring systems do exist, they focus within discrete developmental stages (i.e., early childhood or adolescence) neglecting findings from longitudinal research which repeatedly emphasise the importance of a taking an early life course approach to mental health promotion; one that invests in the formative years of development. Here we describe the design, implementation, and preliminary results of a new population level Australian Comprehensive Monitoring System (CMS) that brings longitudinal science together with epidemiology and a community-based participatory research approach. Through implementation of eight developmentally sequenced community surveys from infancy through young adulthood delivered through existing universal services, high response rates have been achieved, demonstrating feasibility and sustainability of the system. The CMS includes key developmental indicators of physical, relational, emotional, and existential health. Importantly, the CMS also incorporates key indicators of the family, school, peer, digital and community social climates in which young people live and grow. Participating communities lead CMS implementation and are provided with a developmental profile of strengths and difficulties across the early life course. Key learnings include (1) the critical importance of measuring developmentally sequenced indicators of mental health strengths and assets; (2) feasibility of implementation through existing universal services; and (3) the crucial role of community leadership. Next steps include the completion of an interactive dashboard to provide accessible data to community leaders; further psychometric testing to refine the survey instruments; and testing implementation in larger, more socioeconomically diverse communities. The CMS has been developed to guide local level investments in mental health promotion across the early life course, establishing secure foundations for the next generation.