The notion that people make choices and take actions that determine the outcomes of their lives – human agency – is a central principle of the life course paradigm. Unfortunately, conceptualizations of agency, like larger developmental and sociological theories, often assume that agency is limited to individuals who are “developmentally normal.” We draw upon the thought of social scientists and disability scholars, as well as the life history of a woman with intellectual disability, to address the logical, ethical, and empirical flaws of this assumption. To rectify these problems, life course theory and research should pay greater attention to how agency is interwoven with another central principle of the life course paradigm: linked lives. This principle is that an individual’s life cannot be understood in isolation of their interdependencies with other persons. Although human agency and linked lives are discussed as separate principles of the life course, they are not separate in lived experience. We demonstrate that, for all people and at all times in life, human agency is dependent upon interpersonal relationships. It is therefore imperative to examine intersections of agency and linked lives in order to more fully and accurately understand life course dynamics in diverse populations. Human agency is profoundly affected by interpersonal relationships and other social factors. Because agency and linked lives are inseparable, agency cannot be conceptualized as an individual characteristic of ‘independent’ actors. We conclude by discussing how life course research can more fully attend to the relationship between agency and linked lives.