Income is a strong predictor of mortality. However, measuring income is not as simple as it may sound. We investigate how different conceptualizations (individual vs. household) and operationalizations (grouping based on age-specific vs. population income distributions) of income can impact mortality inequality estimates. Drawing on Finnish registry data, we study mortality inequalities by income quintiles during 1995–2017, using two health indicators: life expectancy at 25 and age-specific mortality rates. We compute slope and relative indices of inequalities for the two metrics. While the overall trends are similar, the levels differ considerably across measures. Compared with household disposable income, individual income shows larger life-expectancy differences for males, but smaller for females. For both sexes, classifying individuals on the basis of the population income distributions displays larger life-expectancy differences than of age-specific income distributions. We conclude that researchers should pay attention to the measurement issue in future mortality inequalities studies.