Extra-state war, a conflict between a state and non-state actor outside of the state’s borders, is an understudied phenomenon. In order to begin a discussion on this topic, this paper seeks to understand the factors that affect extra-state war duration. Using literature on interstate and intrastate wars, I hypothesize that military intervention in support of the non-state actor, an equitable distribution of third-party military interventions, and economic intervention in the form of support will increase war duration. I also hypothesize that military intervention on behalf of the state and diplomatic intervention by a third party will decrease duration. I test my hypotheses using a multi-method resource design. First, using quantitative data drawn from extra-systemic wars between 1816 and 2007, I find support for the hypothesis that military intervention on behalf of the non-state actor increases duration. In addition, I find that military intervention on behalf of the state also increases war duration. I supplement my regression analysis with a series of case studies on the Western Saharan War, Cisplatine War, and Mozambican War of Independence. I find that an equitable distribution of military interventions as well as economic support increase duration. Diplomatic intervention, on the other hand, decreases war duration. Taken together, my findings suggest that the various forms of intervention play a crucial role in explaining extra-state war duration.