This paper discusses a three-generational mobility process of how grandparental class influences their grandchildren’s class attainment in Britain. Studies on social mobility are typically limited to the parents-child association. Only a few of studies analysed three-generational class mobility but did not pay much attention on the role of grandchildren’s education, which we take into account in this paper. Based on the data from British Household Panel Survey and Understanding Society, our evidence suggests that a direct association between grandparental class and grandchildren’s class attainment exists independent of parental effects. In structural equation modelling with grandchildren’s education and class, we find grandparental class has a direct and significant impact on grandchildren’s education. For grandsons, such impact accounts for a substantial part of grandparental effects; maternal grandparents continue to have small but significant explanatory power on class outcome even after grandsons’ education and parental resources have been controlled for. In the case of granddaughters, grandparental effects on class attainment are wholly mediated through granddaughters’ education. In sum, this indicates strong persistence of inequalities across three generations in Britain. Without considering the influences of grandparents, the models would underestimate the effects of family origins and overestimate the social mobility rates. Our findings contribute to the understanding of the process of how family advantages are passed on over generations.