Prescribed burning is advocated for the sustainable management of fire-prone ecosystems for its capacity to reduce fuel loads and mitigate large high-intensity wildfires. However, there is a lack of comprehensive field evidence on which to base predictions of the benefits of prescribed burning for meeting either wildfire hazard reduction or conservation goals. Australian eucalypt forests are among the very few forest types in the world where prescribed burning has been practised long enough and at a large enough spatial scale to quantify its effect on the incidence and extent of unplanned fires. Nevertheless even for Australian forests evidence of the effectiveness of prescribed burning remains fragmented and largely unpublished in the scientific literature.We analysed a 52-year fire history from a eucalypt forest region in south-western Australia to quantify the impact of prescribed burning on the incidence, extent and size distribution of wildfires. Quantile regression identified the longevity of the influence of prescribed fire treatments on wildfire incidence and extent. Anomalies in the frequency-size distribution of unplanned fires were identified through a relative risk mapping using kernel density estimates. Changes in the spatial distribution of fuel age were quantified using patch metrics, while generalized additive models were applied to estimate effects of fuel age patterns on the incidence and extent of unplanned fire.Prescribed burning has pronouncedly changed the spatial distribution of fuel age in the study area and has significantly reduced the incidence and extent of unplanned fires. This effect on both the incidence and extent of unplanned fires was minimal for time lags greater than 6 years between fuel treatment and response. When averaged over 6-year periods, the annual extent of prescribed burning explained 24% and 71% of the variation in the mean annual number and extent of unplanned fires, respectively. The incidence of large unplanned fires was significantly less than the long-term average for the region when the annual extent of prescribed fire was at a maximum and significantly more when the annual extent of prescribed fire was at a minimum. Since the 1960s, the length of time sites remain unburned by wildfire has approximately doubled to 9 years. We found that each unit area reduction in unplanned fire required about four units of prescription fire. These findings concur with the observations of experienced field practitioners who identify the 6-year mark as effective at reducing wildfire hazard. Our findings provide strong empirical evidence of the effectiveness of prescribed burning for mitigating wildfire hazard in SW Australian forests. Ongoing research and development is needed to implement managed fire regimes that integrate wildfire mitigation with conservation of biodiversity and other environmental values.