The idea that the population of the United States first became majority urban in 1920 has been repeated so often that it has become “fact,” uncritically reprinted in high school textbooks, academic works, reports of major research organizations, popular urbanist nonfiction, and more. However, this widely accepted “fact” warrants a reevaluation. What does the word "urban" mean and who defined it? Which characteristics distinguish urban space and who devised these criteria? When and why did "urban" become one of the most fundamental demographic and spatial categories by which data on people and places are collected and reported? City Mutable excavates the intellectual origin story of the urban concept by examining the efforts of the United States Census Bureau to define the city for statistical analysis between 1850 and 1930. It argues that the Census Bureau did not merely observe American urbanization but also shaped it by creating the predominant techniques and methodologies for tracking urban growth in the United States. The agency enshrined urban greatness as a measure of national development in the eyes of the American public and its emphasis on the unit of the incorporated municipality incited the late nineteenth- and early twentieth century municipal annexation movement. By highlighting how pro-business organizations like chambers of commerce participated in drafting the census’s metropolitan definitions, it contends that turn-of-the-twentieth-century political, commercial, and industrial interests left an indelible imprint on the census’s urban-rural classification scheme. The Census Bureau’s urban definition continues to guide state and federal funding formulas, inform scholarship and public policy, and direct narratives of American progress—or decay. By unearthing the evolution of this powerful idea, City Mutable prompts readers to question accepted demographic “turning points,” such as the United States population becoming majority urban in 1920 and the world population becoming majority urban in 2007. It encourages academics and policy makers to engage more critically with the constructs employed to study urbanization and opens space for radical and creative reimaginings of the meanings of the word "urban."