Reared in a family active in revolutionary and left-wing causes against the British, Behan at the age of eight began what became a lifelong battle with alcoholism. After leaving school in 1937, he learned the house-painter's trade while concurrently participating in the Irish Republican Army (IRA) as a courier.

Behan was arrested in England while on a sabotage mission and sentenced (February 1940) to three years in a reform school at Hollesley Bay, Suffolk. He wrote an autobiographical account of this detention in Borstal Boy (1958). He was deported to Dublin in 1942 and was soon involved in a shooting incident in which a policeman was wounded. He was convicted of attempted murder and sentenced to 14 years. He served at Mountjoy Prison, Dublin, the setting of his first play, The Quare Fellow (1954), and later at the Curragh Military Camp, County Kildare, from which he was released under a general amnesty in 1946. While imprisoned, he perfected his Irish, the language he used for his delicately sensitive poetry and for An Giall (1957), the initial version of his second play, The Hostage (1958).

Subsequent arrests followed, either for revolutionary activities or for drunkenness, which also forced various hospitalizations. In 1948 Behan went to Paris to write. Returning to Dublin in 1950, he wrote short stories and scripts for Radio Telefis Éireann and sang on a continuing program, Ballad Maker's Saturday Night. In 1953 he began in the Irish Press a column about Dublin, later collected (1963) in Hold Your Hour and Have Another, with illustrations by his wife, Beatrice Salkeld, whom he had married in 1955.

The Quare Fellow opened at the small Pike Theatre, Dublin, in 1954 and was an instant success. A tragicomedy concerning the reactions of jailors and prisoners to the hanging of a condemned man (the “quare fellow”), it presents an explosive statement on capital punishment. The play was subsequently performed in London (1956) and in New York City (1958). The Hostage, however, is considered to be his masterwork, in which ballads, slapstick, and fantasies satirize social conditions and warfare with a personal gaiety that emerges from anguish. The play deals with the tragic situation of an English soldier whom the IRA holds as a hostage in a brothel to prevent the execution of one of their own men. A success in London, the play opened in 1960 off Broadway, New York City, where Behan became a celebrated personality.

Behan's last works, which he dictated on tape, were Brendan Behan's Island (1962), a book of Irish anecdotes; The Scarperer (1964), a novel about a smuggling adventure, first published serially in the Irish Press; Brendan Behan's New York (1964); and Confessions of an Irish Rebel (1965), further memoirs.