O'Neill dramatically transformed American theatre in the early twentieth century. While borrowing structures from Greek tragedy and Shakespeare, he wrote dialogue in everyday American English. This showed that tragic themes could belong not only to kings and heroes but also to ordinary people. In his masterpiece "Long Day's Journey into Night," he depicted drug addiction and family conflict with unflinching honesty, shocking audiences. His plays reflected social inequality and broken dreams in the United States, prompting viewers to examine themselves. These qualities of realism and emotional depth earned him the Nobel Prize in Literature in 1936. His innovations continue to influence Broadway and theatre stages around the world today.

Eugene O'Neill revolutionized American theatre, which had been dominated by melodrama, by introducing psychological realism entwined with symbolism and avant-garde techniques. In his early “Sea Plays” trilogy, inspired by his own seafaring experience, he explored themes of isolation and resistance to fate. During his middle period, works like "Desire Under the Elms" and "The Hairy Ape" employed expressionistic devices, using abstract sets and lighting to externalize characters' inner states. His late masterpiece "Long Day's Journey into Night" heightened autobiographical elements, depicting addiction, illness, and self-deception within a family drama, exposing the dark side of the American Dream. His conception of tragedy centers on “ordinary people bound by inevitable flaws collapsing under the friction between ideal and reality,” thus redefining classical notions of tragedy. The use of silence and tension-filled pauses in his staging later influenced writers such as Harold Pinter and Arthur Kopit. O'Neill's collaboration with the Provincetown Players in the 1920s created a model in which regional theatre nurtures new playwrights, elevating the professional status of American dramatists. These cumulative achievements led to his being awarded the Nobel Prize in Literature in 1936.

Building on Ibsenian realism yet hybridizing Strindbergian dream-play structures and expressionist montage, O'Neill established the prototype of twentieth-century American tragedy. "The Hairy Ape" is notable for theatricalizing the ontological anxiety of the alienated subject in industrial society through personification and rhythmic sound design. His plays deploy meta-textual interludes and elaborate stage directions, simultaneously ensuring multi-layered directorial interpretation and asserting textual autonomy. The late autobiographical cycle experiments with a temporality where Freudian unconscious intersects with Bergsonian durée, positioning it within the modernist project of American literature. Structurally, his contrapuntal repetition of dialogue and strategic silences dislocate speech acts to present ontological void, anticipating Absurdist theatre prior to Sartre. Consequently, he proposed a practice that bridged Broadway commercial theatre and experimental stages, exerting decisive influence on the subsequent canon of Williams, Miller, and Albee.