Internet advertisements have come under tremendous scrutiny recently for their potential to impact electoral outcomes. However, academic research has yet to determine if they have an effect on turnout. This article presents the results of a preregistered field experiment conducted in Dallas, Texas, in partnership with The Dallas Morning News in which individually targeted banner ads were able to generate a statistically significant increase in turnout among Millennial voters in a municipal election. The results show that a combination of information and voting reminder ads was effective, but only for voters in competitive districts. Estimated treatment effects were on par with a telephone mobilization campaign using live callers. These findings contribute to theoretical knowledge about the role of political knowledge and electoral competitiveness in voter mobilization, and offer a new method for testing online advertisements used by political campaigns.