A syndemic has been theorized as a cluster of epidemics driven by harmful social and structural conditions wherein the interactions between the constitutive epidemics drive excess morbidity and mortality. We conducted a mixed-methods study to investigate a syndemic in Soweto, South Africa, consisting of a population-based quantitative survey (N = 783) and in-depth, qualitative interviews (N = 88). We used ethnographic methods to design a locally relevant measure of stress. Here we show that multimorbidity and stress interacted with each other to reduce quality of life. The paired qualitative analysis further explored how the quality-of-life impacts of multimorbidity were conditioned by study participants’ illness experiences. Together, these findings underscore the importance of recognizing the social and structural drivers of stress and how they affect the experience of chronic illness and well-being.