Human-caused decreased fire frequency allows a shift toward woody vegetation in North American grasslands. Since the 1960s, Midwest hill prairies have declined in area due to invasion by Juniperus virginiana L. (eastern redcedar) and encroachment of deciduous woody vegetation. This process has been thought to result from reduced fire frequency but supportive data have not been available. We used tree-ring analysis of J. virginiana to develop chronologies of tree growth and fire scars since the mid-1800s at Fults Hill Prairie Nature Preserve, Monroe County, IL, and used them to understand effects of historical climate and changing fire regimes on loss of prairie habitat. Tree-ring analysis indicated spread of J. virginiana from escarpment habitat into prairie after 1960. Linear regression of time series data from aerial photos projected 80–100% loss of prairie cover before the year 2040. The J. virginiana fire scar chronology revealed a < 2-yr median fire return interval between 1850 and 1959. The percentage of scarred trees (a proxy for fire severity) was greater before 1900, and fire years corresponded to drought years during this period. After 1900, railroad development probably increased local ignitions and spread of fire, decoupling fire from drought. After 1960, J. virginiana invasion and loss of prairie area corresponded to a 5-yr median fire interval as well as increased human population density and loss of railroad ignitions. These changes suggest that hill prairies are highly fire-dependent, and that decoupling from climatic control of their fire regime and reduced fire frequency caused by changing human land use are contributing to their decline. They are also apparently prone to woody invasion due to proximate sources of trees. Juniperus virginiana establishment in prairie followed a 2-yr median fire return interval, and their survival increases with time since fire. As with more mesic eastern grasslands, a < 3-yr fire return interval may be needed to maintain hill prairie vegetation. Restoration of woody vegetation–invaded hill prairies may require supplementary management.