Radon is a human carcinogen and is an important occupational and public health concern. Radiation protection programs, implemented since the 1950s and 1960s, have significantly reduced radon levels in uranium mines and improved the quality of exposure information. Precise quantification of the risk of lung cancer at low levels of radon exposure and the factors that confound and modify this risk are important for occupational radiation protection. This is a joint cohort study of 408 lung cancer deaths and 394,236 person-years of risk from the Czech, French and Canadian male uranium miners, employed in 1953 or later, with cumulative radon exposures < 100 working level months (WLM). Internal Poisson regression models, stratified by cohort, age at risk and calendar year period at risk were used to calculate the linear excess relative risk (ERR) per unit cumulative radon exposure (lagged five-years). The impact of effect modifiers: time since exposure, attained age, and exposure rate were assessed using an exposure-age-concentration model. Finally, sensitivity analyses assessed the confounding effect of unmeasured tobacco smoking on the radon-lung cancer mortality risk estimate. A statistically significant linear relationship between radon and lung cancer mortality was found, with ERR/WLM = 0.017 (95% confidence intervals (CI): 0.009–0.028) at < 100 WLM cumulative radon exposures. In trend analyses, statistically significant risk was observed at cumulative exposures as low as 10–19 WLM, with RR = 1.64 (95% CI: 1.03–2.65, N = 48 deaths). Radon exposures received in more recent periods (5–14 years previously) had the greatest risk of lung cancer mortality. These risks decreased with increasing time since exposure and attained age. No effect of exposure rate, separate from measures of cumulative exposure, was observed at low radon levels. The confounding effect of unmeasured tobacco smoking was small and did not substantially change the radon-lung cancer mortality risk estimates found. These findings provide strong evidence for an increased risk of lung cancer mortality after long-term low radon exposure among Czech, French and Canadian uranium miners. The results are compatible with other studies of miners restricted to low radon levels and residential radon studies. The results suggest radiation protection measures are of significant importance among modern uranium miners with low radon exposure levels.