This article reports findings from a 13‐month ethnographic study of knowledge use and expertise among 78 workers in a U.S. residential treatment center for children. It investigates the question of how youth care workers developed expertise in an organization that did not require graduate professional education and provided little didactic training. It demonstrates how processes of informal apprenticeship allowed some workers to develop locally recognized expertise through working alongside more experienced peers. It analyzes the puzzling initial finding that workers simultaneously attributed expertise to innate “common sense” and reported that they developed this form of expertise through informal on‐the‐job apprenticeship. Taking a cultural perspective on learning, this article conceptualizes youth‐serving organizations as communities of practice in which informal apprenticeship contributes to the development of locally valued forms of expertise and addresses the question of why youth care workers appeared to differ in their ability to make use of opportunities for such learning. It explores implications of these findings for workforce development in youth residential treatment and for social work education in general, suggesting simple methods for maximizing opportunities for situated learning.