Long Day's Journey into Night was written between 1939 and 1941, but was only published and performed in 1956, three years after O' Neil's death. The play was an immediate success and made a crucial contribution to the critical reassessment of the author after the failures of The Iceman Cometh (1946) and A Moon for the Misbegotten (written in 1943, performed in 1947). Long Day's Journey into Night is considered an American classic.

The play is heavily and candidly autobiographical so that members of the O'Neil's family can be easily recognized in the characters of the play. Like O'Neil's own father, James Tyrone is an embittered actor who has failed in his artistic ambitions. His wife Mary, like the playwright's mother, is addicted to drugs. The two brothers struggle against addiction and disillusionment and are clearly placed at the margins of society. Because of this less-than-flattering family portrait, O'Neil had written in his will that he didn't want the play to be published until 25 years after his death. His widow, however, agreed to the publication of the play with Yale University Press.

The social impact of the play was to make controversial issues such as alcholism, drug addiction, family unhappiness and perverse behaviour acceptable topics on the American stage of the conservative 1950s. Like O'Neil's other plays, Long Day's Journey into Night offered a tragic vision of American life through a medium, the theatre, that had long been looked at with suspicion because of the cultural influence of Puritanism. With Long Day's Journey into Night, O'Neil introduced realist representations of social deviancy into American theatre and society . The playwright borrowed his techniques from European authors such as Henrik Ibsen and August Strindberg.