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In response to COVID-19, many scholars and policymakers are urging the U.S. to implement a nationwide system of voting-by-mail to safeguard the electoral process. What are the effects of vote-by-mail programs? In this paper, we provide a comprehensive design-based analysis of the effect of vote-by-mail on electoral outcomes. We collect data from 1996-2018 on all three U.S. states who implemented vote-by-mail in a staggered fashion across counties, allowing us to use a difference-in-differences design at the county level to estimate the causal effect of vote-by-mail programs. We find that: (1) vote-by-mail does not appear to affect either party's share of turnout; (2) vote-by-mail does not appear to increase either party's vote share; and (3) vote-by-mail modestly increases overall average turnout rates, in line with previous estimates. All three conclusions support the conventional wisdom of election administration experts and contradict many popular claims in the media.