The COVID-19 crisis has generated interest in all-mail voting (AMV) as a potential policy solution for avoiding in-person elections. However, the quality of AMV implementation has varied greatly across states, leading to mixed results in previous research. We exploit the understudied 2014 implementation of AMV in Colorado to estimate the effect on turnout for all registered voters, along with age, racial, education, income and wealth, and occupational subgroups. Using large voter file data and a difference-in-differences design within individuals, we find a positive overall turnout effect of approximately 8 percentage points-translating into an additional 900,000 ballots being cast between 2014 and 2018. Effects are significantly larger among lower-propensity voting groups, such as young people, blue-collar workers, voters with less educational attainment, and voters of color. The results suggest that researchers and policymakers should look to Colorado's AMV approach as an effective model for boosting aggregate turnout and reducing voting disparities across subgroups.