Thomas was born at home in the Uplands district of Swansea, Wales, on October 27, 1914, the second child and only son of middle-class parents. His sister Nancy was nearly nine years older than he. His father was a schoolmaster in English at the local grammar school. Though considered a cold and bitter man who resented his position as a teacher, the elder Thomas's love for literature encouraged a similar devotion in his son. Thomas feared, respected, and deeply loved his father, and in some sense his life appeared to be an attempt to realize his father's frustrated dream of being a great poet. In contrast to his father, Thomas's mother was loving, overly protective, and inclined to overindulge her son. Even at the end of his life, she found no fault in his public behavior and the drinking habits which ultimately led to his death.
Thomas enjoyed his childhood in Wales, and his work in later years would reflect a desire to recapture the relatively carefree years of his youth. A generally undistinguished student, Thomas entered the Swansea Grammar School in 1925. In 1931 he left school to work for the South Wales Daily Post in Swansea. He would later say that his real education came from the freedom he was given to read anything in his father's surprisingly well-stocked library of modern and nineteenth-century poetry and other works. Following his resignation from the paper early in 1933, poetry became Thomas's primary occupation. By all accounts, he was not a successful news reporter: he got facts wrong, and he failed to show up to cover events, preferring instead to loiter at the pool hall or the Kardomah Cafe. During the early 1930s Thomas began to develop the serious drinking problem that plagued him throughout the remainder of his life. He also began to develop a public persona as a jokester and storyteller. However, his notebooks reveal that many of his most highly regarded poems were either written or drafted during this period and that he had also begun to experiment with short prose pieces. In May of 1933 his poem "And Death Shall Have No Dominion" was published in the New English Weekly, marking the first appearance of his work in a London journal, and in December of the following year his first poetry collection, 18 Poems (1934), was issued. During this period he established a lifelong pattern of travel between London and some rural retreat, usually in Wales. As the decade progressed he gained increasing recognition for both his poetry and his prose.
In the summer of 1937 Thomas married Caitlin Macnamara, a young dancer of Irish descent whose Bohemian lifestyle and behavior rivaled Thomas's own. For the next twelve years the couple led a nomadic and financially difficult existence, staying with friends, relatives, and a series of benefactors. The stories later collected in Portrait of the Artist as a Young Dog (1940) were written primarily during the couple's stay in the Welsh coastal village of Laugharne in late 1938 and early 1939. Too frail for active military service and needing to support himself and his wife, Thomas took work writing scripts for propaganda films during World War II, at which time he also began to participate in radio dramas and readings for the British Broadcasting Corporation (BBC). His financial burdens increased during this time. In January 1939 Thomas's first child, a son named Llewelyn, was born. Daughter Aeron followed in March 1943. Thomas emerged from the war years a respected literary figure and popular performer; however, his gregarious social life and the excessive drinking it encouraged seriously interfered with his writing. Seeking an environment more conducive to poetic production, Thomas and his family returned to Laugharne in 1949.
During the early 1950s Thomas wrote several of his most poignant poems, including "Do Not Go Gentle into That Good Night" and "Lament." Nevertheless, fearing that his creative powers were rapidly waning, and seeking to avoid the pressures of writing, he embarked on a speaking tour of the United States in the spring of 1950. During the final years of his life he traveled to the United States four times, each time engaging in parties and readings in and around New York City, followed by readings and more celebrations at numerous universities throughout the country. Thomas's personal charisma and self-described public reputation as a drunkard, a Welshman, and a lover of women seemed to serve only to enhance his standing in literary circles. His fourth and final American tour began on October 19, 1953, and ended with his death from a massive overdose of alcohol on November 9.