Kenneth Slessor's family moved from Orange to Sydney in 1903. Slessor was a voracious reader and began writing poetry as a child. He was educated at Sydney Church of England Grammar School (Shore) where he edited the school magazine. He gained first class honours in English in the Leaving Certificate in 1918, then learned typing and shorthand before joining the Sydney Sun in 1920 as a cadet journalist. Slessor's first publication was a poem about war in the Bulletin in 1917 and in 1918 he won the Victoria League's prize for a patriotic poem 'Jerusalem Set Free'. More poems followed in literary magazines, but he had not yet employed the modern forms for which he is known. In 1923 he collaborated with Norman and Jack Lindsay on the short-lived literary magazine, Vision. His edition (with Jack Lindsay) of Poetry in Australia also appeared that year, the first of several similar anthologies he would edit over the next forty years.
After a period in Melbourne in 1924-1925, writing for the Herald and sub-editing Punch, Slessor returned to Sydney. He joined the staff of Smith's Weekly in 1927, holding the position of editor from 1935 to 1940. In the Weekly he published light verse and regular articles and reviews. He describes the period as 'the happiest chapter of my existence'. In 1940 Slessor was appointed official war correspondent. He served in North Africa, Greece, Crete and Syria until 1943, and in Papua and New Guinea in 1943-1944. After the war he returned to editorship of the Sun.
Journalism remained Slessor's dominant occupation throughout his life, but in the years between 1924 and 1948, he wrote some of the most important poems in the history of Australian literature. Slessor's first book appeared in 1924, followed by six more volumes over the next twenty years. His poetry explores the themes of art, nature, beauty and time in the Romantic-Symbolist tradition, attracting the widely held view that he ushered in the modern era of Australian poetry. His poem 'Five Bells' is widely regarded as his most important poem, and the war poem 'Beach Burial' is frequently anthologised. In addition to his poetry, he wrote a large amount of highly regarded literary criticism.
Slessor published little or no new work after 1948, but his reputation continued to grow. For the next thirty years he continued to work as a journalist and remained involved in Australian literature, editing Southerly between 1956 and 1962, and serving on the advisory board of the Commonwealth Literary Fund. An opponent of censorship, he agreed to serve on the National Literature Board of Review in 1967. He was an active and influential participant in Sydney's literary culture, strengthened by his presidency of the Sydney Journalists Club between 1956-65.
Slessor was married twice and had one child. Suffering from heart disease and other health problems late in life, he died in Sydney in 1971.