How does a persistent change in mortgage rates affect homeownership, home prices, construction, and household formation? We study the government-led revolutions in mortgage financing that took place in the U.S. between 1933 and 1940, which created a national mortgage market facilitating mortgage capital to move from the financial centers to the rest of the country. By digitizing city-level census data and a new sample of loan-level data, we show that differences in mortgage rates across cities went from nearly 300 basis points to just over 80 in only six years. In the decades following the housing policies, cities where mortgage rates declined as a result of the integration of mortgage markets saw higher growth in rates of homeownership and housing construction. House prices moved only modestly, and we estimate highly elastic supply curves, in stark contrast with estimates from more modern data. Households that, due to these policies, faced lower costs of homeownership in their youth had more children and had them earlier.