Facing a fiscal threat from mass suburbanization, Sun Belt cities expanded their municipal boundaries nearly six-fold between 1945 and 2000, while Northern cities, constrained by state annexation laws, saw little expansion. Using newly digitized data on city boundaries, we estimate that the average expansion increased municipal population by over 35%, adding whiter and higher-income neighborhoods than the pre-existing core. Through a stacked difference-in-differences design comparing annexing cities to similar non-annexing cities, we evaluate how boundary expansions impacted municipal finance and public good provision. While total revenues and expenditures increased after annexation, per capita levels declined by roughly 25%, with the largest declines in current expenditures and labor-intensive services like fire and policing. However, proxies for public good provision show no decline post-annexation, suggesting cities maintained service quality by leveraging economies of scale in high fixed-cost sectors. Despite contemporary claims that annexation would spur broader economic growth, we find no evidence of increased county-level employment.