Objectives The deleterious health effects of long working hours have been previously investigated, but there is a dearth of studies on mortality resulting from accidents or suicide. This prospective study aims to examine the association between working hours and external-cause mortality (accidents and suicide) in Korea, a country with some of the longest working hours in the world. Methods Employed workers (N=14 484) participating in the Korean National Health and Nutrition Examination Survey (KNHANES) were matched with the Korea National Statistical Office's death registry from 2007-2016 (person-years = 81 927.5 years, mean weighted follow-up duration = 5.7 years). Hazard ratios (HR) for accident (N=25) and suicide (N=27) mortality were estimated according to weekly working hours, with 35-44 hours per week as the reference. Results Individuals working 45-52 hours per week had higher risk of total external cause mortality compared to those working 35-44 hours per week [HR 2.79, 95% confidence interval (CI) 1.22-6.40], adjusting for sex, age, household income, education, occupation, and depressive symptoms. Among the external causes of death, suicide risk was higher (HR 3.89, 95% CI 1.06-14.29) for working 45-52 hours per week compared to working 35-44 hours per week. Working >52 hours per week also showed increased risk for suicide (HR 3.74, 95% CI 1.03-13.64). No statistically significant associations were found for accident mortality. Conclusions Long working hours are associated with higher suicide mortality rates in Korea.