Ambient benzene is a volatile anthropogenic pollutant and known carcinogen associated with industrialization and urbanization. Benzene is a natural constituent of petroleum, so cities, which concentrate combustion through industrial activity, transit and heating, generate a great deal. In addition to causing cancer, theory also predicts that benzene may chronically affect the human brain, even at a low level (<5 µg m−3). In this study, we estimated associations of ambient benzene exposure before 2010 with brain disorders (261,909 participants) and brain imaging phenotypes (23,911 participants) in urban residents in the UK (enrolled during 2006–2010 and followed up to 2022). The results show that ambient benzene (per interquartile range increment of 0.30 µg m−3) is associated with elevated risks of dementia (hazard ratio, 1.18; 95% confidence intervals, 1.09 to 1.28), major depression (1.09; 1.03 to 1.14) and anxiety disorder (1.16; 1.10 to 1.22). Neuroimaging analysis highlighted the associations with brain structures, including the thalamus and the superior temporal gyrus. This study provides population-level evidence of the effect of ambient benzene on brain disorders in urban populations, critical for risk assessments, air quality and health guidelines, and sustainable-development efforts as the world urbanizes.