History is replete with racially targeted voter suppression, yet empirical evidence on the consequences of specific efforts is rare. We study electoral reforms introduced by Prime Minister Cecil Rhodes and others in the Cape Colony (South Africa) at the turn of the 20th century. Using the complete voter roll for 1903 we estimate that over the preceding decade these reforms disenfranchised between 10,320 and 15,610 mostly Black and mixed-race voters, likely compounding into the future. Without suppression the electorate in 1903 would have been 7.6 to 11.5% larger, with the number of voters of colour 50 to 75% higher, magnitudes that we show could have altered election outcomes. Using unique features of the roll we quantify three tools -- a socioeconomic lever, a spatial lever, and a discretion lever -- that constituted the basic architecture of large-scale racially targeted voter suppression, culminating in mass racial disenfranchisement under apartheid.