Lifespan inequality refers to the variation in the age at which people die or the uncertainty surrounding the time of their death. This study investigates the patterns of lifespan inequality at the state level in the United States between 1959 and 2018, utilising Theil's entropy index. We also explore the demographic and socioeconomic factors associated with lifespan inequality using a Panel-Corrected Standard Errors (PCSE) model. We observe a strong and statistically significant negative correlation between life expectancy and lifespan inequality at the state level overall and for both males and females. In terms of demographic and socioeconomic factors, the percentage of individuals who have completed high school and college education, the percentage of the Hispanic population, the number of physicians, the percentage of individuals under 65 with insurance, and population growth are all negatively associated with lifespan inequality. Moreover, there is a positive association between lifespan inequality and the rates of violent crime, CO2 emissions per capita, and cigarette smoking. Our results reiterate that policies aimed at tackling disparities in socioeconomic position could also serve as useful strategies for addressing health disparities.