Intergenerational disparity in income and health violates the norm of equal opportunity and deserves the attention of researchers and policy makers. To understand changes in intergenerational disparity, we created the intergenerational mobility index (IMI), which can simultaneously measure changes in income rankings and in health outcomes across two generations. We selected obesity as one health outcome to illustrate the application of IMI due to its severe health and financial consequences for society and the significant changes in the distribution of obesity across income groups. Although obesity has increased in all income groups in the last four decades, higher income groups have tended to have a faster increase in obesity, which has reduced the disparity in obesity across income groups. The strength of our intergenerational approach within families is to control the genetic influence, which is one of the strongest determinants of obesity. The decomposition of the IMI illustrates that it captures changes in obesity distribution (holding constant income rankings between generations) and changes in income rankings (holding constant the obesity distribution across generations), simultaneously. We used the data of the Panel Study of Income Dynamics (PSID), which have been collected since 1967, is the longest longitudinal survey in the U.S. The PSID surveyed respondents’ height and weight were recorded in 1986 and from 1999 to 2007. We selected respondents from 1986 as the parental generation and respondents from 2007 as the adult children’s generation. To make the adult children’s body weight status and income comparable to their parents’, we stratified the analysis by gender. For the pairs of fathers and adult sons, we found the intergenerational disparity in overweight, a less severe indicator of excessive fatness, across income was decreasing. This was partially due to the up-swing in the adult children’s income status. For the pairs of mothers and adult daughters, we found a similar decrease in socioeconomic disparity in obesity. However, decomposition of the IMI indicated that changes in income distributions between mothers and adult daughters contributed smaller effects than that between fathers and adult sons. Our study has demonstrated that the IMI and its decomposition are useful tools for analyzing intergenerational disparity in income and health.