The transition from limited to open access societies is a fundamental driver of economic development. This paper provides empirical insights into this institutional transformation by investigating early and high Tang China (618–906). Using a dataset comprising 1,272 marriages ranging from 618 to 755, we find that Empress Wu’s rise to power in 674 resulted in a positive effect on upward mobility. Males from poor and common clans who entered marriage after 674 were more likely to wed into elite clans and consequently became lineage elites. This surge in inter-class marriages primarily triggered by Empress Wu’s expansion of national civil examinations to strengthen her legitimacy, which increased the likelihood of men from poor and common clans succeeding in the exams. These knowledge elites leveraged their academic accomplishments to marry daughters from elite clans, leading to the convergence of lineage elites and knowledge elites. This paper, therefore, examines the rise of meritocracy, a classical form of an open access order that plays an important role in increasing social mobility, in a causal context.