Large scale migrations, especially involuntary ones, can have a sudden and substantial impact on the demographics of both sending and receiving communities. The partition of the Indian subcontinent in 1947 resulted in one of the largest and most rapid population exchanges in human history. We compile census data to estimate its impact on a district’s educational, occupational, and gender composition. Comparing neighboring districts within a state better isolates the effect of the migratory flows from secular changes. We find large effects within four years. With migrants typically more educated than non-migrants, inflows into a district raised its literacy levels (by 12-16% more than less affected districts in India and Pakistan) while outflows reduced it (by 20% in Pakistan). The effects are asymmetric across regions. With relatively less land vacated by those who left Indian Punjab, Indian districts with large inflows saw a decline of 70% in the growth of agricultural occupations. In contrast, Indian districts with large outflows saw an increase of 56% in the growth of the fraction of agriculturists. Initial differences in migrant characteristics also meant there were large net effects even when a district saw similar two-way flows. Along the gender dimension, Indian districts with large outflows saw a greater increase in gender balance (the percentage of males fell); the corresponding inflows into Pakistani districts also improved the gender balance. While Pakistani in-migrants had higher male ratios relative to the communities they left, these ratios were lower compared to the communities they migrated to. Given the partition was along religious lines - with Muslims leaving India and Hindus and Sikhs leaving what became Pakistan and Bangladesh – it increased religious homogenization within communities. However, our results suggest that this was accompanied by increased educational and occupational differences within religious groups. We hypothesize that these compositional effects, in addition to an aggregate population impact, are likely features of involuntary migrations and, as in the case of India, Pakistan, and Bangladesh, can have important long-term consequence.