Many counties in the U.S. federalist system have morphed from a limited role in service delivery to a workhorse for municipal-style local government. They also facilitate development and sprawl, helping to shape development patterns of the modern fragmented metropolis. Why do counties accommodate development demand that deviates from long-term land-use plans intended to prevent sprawl? Utilizing panel data of county land-use changes in Florida, this study finds evidence that the decisions are shaped by both external competition for growth and internal institutional incentives. Fragmentation fuels more leapfrog development patterns on the urban fringe. Horizontal fragmentation encourages counties to compete for development, whereas vertical fragmentation via special districts facilitates such development through provision of services and reducing pressure for public officeholders to raise taxes. However, these fragmentation effects are also influenced by modernized institutions in counties such as home-rule charters and form of government.