Through their nonpartisan voter registration and get-out-the-vote (GOTV) activities, nonprofit human service organizations play a critical role in promoting a more inclusive democracy by engaging low-income citizens and marginalized populations in the voting process. Previous studies of nonpartisan voter mobilization have focused on the effectiveness of door-knocking and phone calls by large community organizing groups as a strategy for increasing voter turnout. However there have been no studies to date examining the effectiveness of offering voter registration and other forms of voting support at the service delivery site, a strategy described as "agency-based" voter engagement. Using a quasi-experimental design, this study addresses this gap in the literature by examining the effects of nonprofit voter mobilization efforts on increasing voter turnout. We use voter turnout data from 505 clients of seven nonprofit human service organizations in the city of Detroit collected after the 2010 elections. The findings yield strong evidence that nonprofits’ agency-based voter mobilization efforts are effective. We find that for each voting-related contact a client receives from his or her service agency, the likelihood of turnout increases by 11.1 percentage points, even after controlling for other factors known to influence voting behavior. Moreover, we find that the most effective forms of voting-related contacts are voter registration assistance and personal voting reminders. We conclude by examining the implications of these findings for nonprofits’ voter engagement efforts.