Subjective and sensuous, Thomas' first collection announced the arrival of a strikingly individual voice and was praised by a number of established poets, notably Edith Sitwell. Although sometimes obscure, the poems established Thomas' main themes of sex, death, religion and childhood innocence. His reputation was consolidated by the publication of Twenty-Five Poems in 1936 and The Map of Love in 1939. Thomas' poems appealed to the heart rather than to the head, stressed the importance of sound and rhythm and combined sexual and religious imagery. His prose was more realistic, and his autobiographical Portrait of the Artist as a Young Dog (1940) contained humorously-observed scenes from his Welsh upbringing.

Witty and sociable, in London Thomas became famous for his exuberant lifestyle, his poverty and his thirst. In 1937, he married Caitlin Macnamara and the couple featuring prominently in bohemian circles before and during World War 2. Thomas was exempted from military service during the war, but wrote powerfully about such subjects as the bombing of London. He has been called Britain's first civilian war poet, with poems such as Ceremony after a Fire Raid movingly describing the death of a child in a V-2 rocket attack. He also wrote film scripts and worked for the BBC as a writer. His war poems were collected in Deaths and Entrances (1946) and they reveal Thomas as a religious poet of considerable power.

The volume also contains poems that celebrate childhood experience in visionary terms, as in Fern Hill and Poem in October. These elegiac poems contrast violently with the poet's reality at that time, for Thomas was now finding his increasingly alcoholic London existence impossible to maintain and in 1947 he experienced a breakdown. The historian A J P Taylor provided him with a cottage in Oxford. Then, in 1949, Thomas returned to Wales, having been offered a cottage at the seaside town of Laugharne. The next year saw his first tour of the United States.