Background <p>We examined trends in life expectancy at birth, modal age at death, and lifespan variation in South Korea from 1970 to 2023, and evaluated hypotheses regarding mortality compression and shifting in relation to increases in life expectancy at birth.</p> Methods <p>Using annual sex-specific complete life tables from Statistics Korea, we fitted the Siler model separately by sex and year. We estimated life expectancy at birth and compared the value with the modal age at death. Lifespan variation was evaluated using multiple indicators, including the standard deviation, life disparity, Theil index, and Gini coefficient. We conducted decomposition analyses to quantify the contributions of shifts in the modal age at death and in mortality compression to annual gains in life expectancy at birth.</p> Results <p>Life expectancy at birth and the modal age at death increased steadily for both sexes over the study period. However, while the rate of increase in life expectancy at birth slowed after 2000, gains in the modal age at death accelerated over the same period. Lifespan variation indicators declined in both sexes, but their downward trend slowed or stagnated after 2000, particularly among women. Decomposition analyses revealed that, since the 2000s, increases in the modal age at death have been the primary driver of gains in life expectancy at birth.</p> Interpretation <p>Over time, the increase in life expectancy at birth in South Korea has become increasingly consistent with the shifting hypothesis. Efforts should be made to ensure that longevity gains are equitably shared across the population. Monitoring of lifespan variation indicators across subpopulations and identifying their underlying determinants across is essential.</p>