Denmark is widely regarded as having among the highest levels of intergenerational educational mobility in the world. However, most research on educational mobility in Denmark studies cohorts who were born after the major compulsory schooling reforms and the rise of the comprehensive welfare state in the 1950s and 1960s. In this paper, we draw on both survey and administrative register data to study educational mobility trends among Danes born across the entire 20th century. While mobility was extremely low in the first half of the 20th century, it increased dramatically for cohorts born during the 1950s and 1960s when major schooling reforms significantly lifted the lower parts of the schooling distribution. However, as the educational expansion shifted from secondary education to college and university degrees for cohorts born during the 1970s and 1980s, educational mobility has been rapidly declining. For the most recent cohorts born in the late 1980s, educational mobility levels are similar to those experienced by cohorts born in the mid-1940s and to the levels reported for the U.S. Our findings thus suggest that educational expansion is strongly linked to changes in educational mobility in Denmark: Lower tail compression is associated with increasing mobility, whereas upper tail expansion is associated with decreasing mobility. Further analyses that draw on the rich administrative registers indicate that, as the educational system expanded, schooling came to proxy different aspects of human capital. Whereas the compression at the lower tail reduced the correlation between education and cognitive skills by 25 percent, the upper tail expansion strengthened the correlation between schooling and crime, earnings, marriage rates, and health, indirectly suggesting an increasing role of noncognitive skills. We discuss how our results feed into discussions on cross-country and cross-regional variation in social mobility.