Many accounts of Anglo-American modernist fiction presuppose its postwar termination and the consequent anachronism of its commitment to autonomous artistic form—a commitment that is now often understood as a merely sociological matter of taste and, as such, a dissimulation of social reality. This dissertation challenges these positions. Building on recent reappraisals of aesthetic autonomy, I demonstrate how the modernist commitment to art’s autonomy can be understood not only in more formally rigorous and less sociological terms than are conventional but also in more historically capacious terms. I explore how a modernist principle of aesthetic autonomy emerges in antagonism to fin-de-siècle processes of marketization and persists today within fiction whose circulation is subsumed by the market. I base this expanded periodization of modernist fiction on an analysis of how works can assert the autonomy of their aesthetic meaning in mass-market contexts where this autonomy has not been analyzed as it has been within the market-insulated contexts of canonical modernism. Taking up key works of Anglo-American fiction whose modernist form has been neglected or misunderstood, I show how these works assert aesthetic autonomy through and against the way they must fashion themselves for mass-market circulation and, in this way, embody a form of realism rooted in irony. What I call the “market architecture” of modernist fiction consists of medium-specific conceptual structures that transform the external, instrumentalizing pressures of market demand into internal, aesthetic problems of self-legislating form. By analyzing the structuring presence of market architecture in forms like popular genre fiction, experimental novels about race, political performance art, and the contemporary sequel novel, I show how cultural problems and projects often deemed antithetical to autonomous art can actually be intrinsic to it.