ImportanceImproving child and adolescent well-being is a critical public health goal, yet monitoring of this measure at the national level remains limited. Composite indices aggregate existing indicators from population data sources, but these indices currently have weaknesses that may inhibit widespread use.ObjectiveTo apply a novel, more child-centric index method to document changes in overall child and adolescent well-being in the US from 2000 to 2019, assess which states and racial and ethnic subpopulations experienced the greatest inequities in well-being, and identify the specific components associated with changes in the index over time.Design, Setting, and ParticipantsThis cross-sectional study applied the Child and Adolescent Thriving Index 1.0 to population-level data from 2000 to 2019 from several data sources. The area-based sampling frame for each of the component data sources allowed for nationally representative estimates for every year of the study period. The indices for every state and by race and ethnicity were also calculated. Due to the scope and breadth of the index components from across the life course, the Child and Adolescent Thriving Index 1.0 is intended to approximate the well-being of persons up to age 17 years. Data were analyzed from June 7, 2021, to March 17, 2022.ExposuresTime in years.Main Outcomes and MeasuresThe Child and Adolescent Thriving Index 1.0 is a weighted mean of 11 indicators intended to proxy well-being. The index comprises 11 components: non–low birth weight in neonates, preschool attendance in children aged 3 to 4 years, reading proficiency in fourth-grade students, math proficiency in eighth-grade students, food security in children younger than 18 years, general health status, nonobesity in high school students, nonsmoking in adolescents aged 12 to 17 years, non–marijuana use in adolescents aged 12 to 17 years, high school graduation in young adults aged 18 to 21 years, and nonarrest rate in children aged 10 to 17 years. The index ranges from 0 to 1, with 0 indicating minimum and 1 indicating maximum possible well-being at the population level.ResultsThe Child and Adolescent Thriving Index 1.0 was applied to data from 12 320 national, state, and racial and ethnic population-level estimates. Over the study period, the Child and Adolescent Thriving Index 1.0 score increased from 0.780 points in 2000 to 0.843 points in 2019. Despite some convergence in geographic and racial and ethnic disparities, inequities were still present in 2019 in the South (−0.021 points) compared with the Northeast and among American Indian or Alaska Native (−0.079 points), Black (−0.053 points), and Latinx (−0.047 points) children and adolescents compared with White youths. Index components most associated with the overall increases in index scores of well-being were high school graduation rate (+0.028 units) and nonsmoking in adolescents (+0.022 units), amounting to 80.6% of the total increase.Conclusions and RelevanceResults of this study suggest that child and adolescent well-being scores increased from 2000 to 2019, but substantial work remains to address persistent inequities across states and racial and ethnic populations. The newly developed Child and Adolescent Thriving Index 1.0 may be used in future work to evaluate which public policy types (economic, social, health care, housing, or education) are associated with higher levels of well-being.