Prior to the COVID-19 pandemic, some American states had transitioned to universal voting-by-mail, where all registered voters receive a mail ballot. But due to the pandemic, universal voting-by-mail was suddenly used in a larger number of states in2020. Here we study a unique situation in which registered voters in some legislative districts in Los Angeles County were subjected to universal voting-by-mail in theMarch 2020 primary. Using difference-in-differences and geographic boundary-based designs on individual-level records, we take advantage of this within-jurisdiction situation to estimate the causal effects of universal voting-by-mail on voter turnout and on who votes. Our results indicate that voter turnout increased by 3 to 4 percentage points for voters who do not automatically receive a mail ballot, and the increase is generally larger for registered partisan voters than those without a party affiliation.