The World Health Organization (WHO) estimates that 140 million individuals are at risk from consumption of drinking water containing arsenic at concentrations above the WHO guideline value of 10 μg/l. Arsenic mitigation is considered to be the most effective way to prevent arsenic related diseases. After joining the European Union, Hungary implemented a Drinking Water Quality Improvement Programme (DWQIP) to reduce levels of arsenic in drinking water below the WHO guideline value. But what impact did this have on health? We estimated the change in lifetime excess skin, lung, and bladder cancer risks and mortality from ischaemic heart disease (IHD) associated with chronic arsenic intake among those exposed before (2004-2007) and after (2014-2017) the implementation of DWQIP. A population-based risk assessment approach was used to assess lifetime excess cancer risk applying two scenarios for lung and bladder cancers. The economic benefits of the DWQIP were estimated by the combination of cost of illness and value per statistical life methods. Compared to the period before the DWQIP, its implementation was associated with a significant reduction in arsenic in drinking water [median: 3.0 μg/l interquartile range (IQR): 1.5-12.0 μg/l to median: 2.15 μg/l IQR: 1.0-5.79 μg/l]. The two scenarios were estimated to be associated with 225.2 and 35.9 fewer cancer cases each year. The number of annually prevented IHD deaths was estimated to be 88.9. It was estimated that the benefits of the DWQIP will outweigh its costs. We conclude that reducing arsenic levels in drinking water to 10.0 μg/l resulted in significant health and economic benefits. Our study goes beyond the existing research, offering both new insights into the impact of arsenic mitigation and providing a methodological template for similar studies in the many parts of the world that have yet to reduce arsenic exposure.