Existing research reveals many of the ways pro-government militia (PGM) shape civil violence but overlooks how the ethno-political ties between the state and a PGM might influence these effects. We argue that co-ethnic militia (i.e., groups composed of the ruling elite’s ethnic kin) are relatively loyal irregular forces that multiply state military capacity. The greater loyalty of co-ethnic groups mitigates principal–agent problems but further polarizes ethnic communities, and as a result, co-ethnic PGMs are likely to be associated with longer and more intense civil conflict. We test this argument on a global sample of cases from 1989 to 2007 using new data capturing the ethnic ties of all PGMs. The results support our claims that co-ethnic militia are associated with more intense and longer civil conflict.