An early career as a theatre director had a fundamental, life-long impact on film director Joseph Losey, a factor consistently underestimated in critical and academic assessments of his filmmaking. Losey's stage work in 1930s New York bore the imprint of some of the early twentieth century's major theatre practitioners. He would translate their ideas into a distinctive cinematic language. This study regards Losey's film output as a cohesive whole, challenging commentators' habitual practice of dividing the director's career into discrete, often incongruous, chronological phases; in place of a compartmentalised view, the thesis articulates specific influences which are consistently reflected in Losey's cinema spanning five decades. The thesis outlines Losey's career in theatre, and explores his theatrical influences, especially Bertolt Brecht and Vsevelod Meyerhold. Close formal analysis of films from Losey's 37-year film career focuses on his mise en scene, his use of vertical space and of cinematography, lighting, sound and editing. Archive material from the British Film Institute and from Losey's alma mater, Dartmouth College in the United States, supports the film analysis. The thesis also examines his work with actors, and with playwrights, particularly Harold Pinter, and on Losey's adaptations of Brecht's Galileo and Ibsen's A Doll's House. Losey is here established as a hybrid figure whose highly personal screen style can be interpreted through his adherence to specific, well-established stage concepts. This research outcome points to revised evaluations of the work of Losey's contemporaries who made similar transitions from 1930s theatre in the United States into filmmaking.