How does the place where someone grows up affect their political behavior in adult life? To answer this question, we track households that relocate to assess the extent to which individuals’ probability of registering, voting, and affiliating with a political party mirrors the average behavior in their childhood neighborhoods. Linking young people in nationally comprehensive voter files to external data on their parents' addresses and moves, we reconstruct voters’ childhood location histories. With these data, we estimate the causal effects of childhood environment on future political behavior by comparing the participation and partisanship of individuals who moved from similar origins to similar destinations before turning 18, but at different ages. We show that children who move at a younger age and spend more time in their destination county are more influenced by the average voting behavior in that county. Spending one’s entire childhood (18 years) in a county where permanent residents end up 10 percentage points more Republican (Democrat) makes voters 1.4 (1.8) percentage points more likely to register as a Republican (Democrat) themselves. The influence of one’s childhood environment on participation and registration is slightly larger still.