To date, research on sex differentials in lifespan variation and life expectancy has mainly been conducted in Western countries and there is a dearth of data from South Korea. This study aimed to further the understanding of mortality transition and life expectancy in South Korea, and the associated trajectories of age-at-death variation, through an analysis of life disparity by gender. Using complete life tables for South Korea for 1970–2015, sex differentials (female–male differences) in life disparity and life expectancy at birth were estimated, and sex differentials in life expectancy were decomposed by age and cause of death. The results showed that sex differentials in life expectancy at birth have not reduced significantly in the last 45 years (1970: 7.1 years; 2015: 6.2 years). Life disparity has reduced more rapidly for females than males, and the difference increased from −0.1 year in 1981 to −1.6 years in 2015. Sex differentials in life expectancy and life disparity in South Korea were higher during 1970–2015 than in several Western countries with high life expectancy. The elderly age group (60 and above) contributed 50% of the total sex difference in life expectancy at birth in 1970, and this increased to 70% in 2015. The contribution of the age group 15–59 years reduced significantly over the period. Decomposition of life expectancy at birth by cause revealed that diseases of the circulatory system (2.2 years), followed by external causes (1.3 years), were the most important causes of the sex differences in life expectancy at birth in 1983, and in 2015 neoplasms (2.2 years) and external causes (1.1 years) explained half of the total sex differences. There has been a significant shift in the age-specific pattern of the contribution towards each cause of death. Overall, sex differentials in life disparity and life expectancy at birth have remained significant in South Korea in the last 45 years.