In this paper, we study the effects of immigration on natives' marriage, fertility, and family formation across US cities between 1910 and 1930. Instrumenting immigrants' location decision by interacting pre-existing ethnic settlements with aggregate migration flows, we find that immigration raised marriage rates, the probability of having children, and the propensity to leave the parental house for young native men and women. We show that these effects were driven by the large and positive impact of immigration on native men's employment and occupational standing, which increased the supply of "marriageable men". We also explore alternative mechanisms − changes in sex ratios, natives' cultural responses, and displacement effects of immigrants on female employment − and provide evidence that none of them can account for a quantitatively relevant fraction of our results.