The influence of ethnicity on vaccination uptake in urban slums in Kenya is largely unknown. We examined the disparities in timeliness and coverage of measles vaccination associated with ethnicity in the Korogocho slum of Nairobi. The study used data from the Maternal and Child Health component of the Urbanization, Poverty and Health Dynamics Research Programme undertaken in the Korogocho and Viwandani slums by the African Population and Health Research Center from 2006 to 2012–2010. Vaccination information was collected from children recruited into the study during the first year after birth, and a sub-sample of 2,317 who had been followed throughout the period and had the required information on measles vaccination was included in the analysis. Cox regression analysis was used to determine the association of ethnicity with delayed measles in the slum. We found significant disparities in the coverage and timeliness of measles vaccination between the ethnic groups in Korogocho. The Luhya and minor ethnic groups in the slum were more likely than the Kikuyu to have delayed measles vaccination. Ethnic groups with a high proportion of children with delayed measles vaccination need to be targeted to address cultural barriers to vaccination as part of efforts to improve coverage in urban slums.