In September 1915, during World War I, Blunden was commissioned as a second lieutenant into the British Army’s Royal Sussex Regiment. He was posted to the 11th (Service) Battalion (1st South Down), Royal Sussex Regiment, a Kitchener’s Army unit that formed part of the 116th Brigade of the 39th Division in May 1916, two months after the battalion’s arrival in France. He served with the battalion on the Western Front to the end of the war, taking part in the actions at Ypres and the Somme, followed in 1917 by the Battle of Passchendaele. In January 1917, he was awarded the Military Cross (MC) for “conspicuous gallantry in action”.

Blunden was married three times. While still in the army, he met and married Mary Daines in 1918. They had three children, the first of whom died in infancy. They divorced in 1931, and in 1933, Blunden married Sylva Norman, a young novelist and critic. That marriage, which was childless, was dissolved in 1945. The same year, he married Claire Margaret Poynting (1918–2000), one of his former pupils; they had four daughters. While in Japan in the summer of 1925, he met Aki Hayashi, and he began a relationship. When Blunden returned to England in 1927, Aki accompanied him and would become his secretary. The relationship later changed from a romantic one to a platonic friendship, and they remained in contact for the rest of her life.

Blunden left the army in 1919 and took up the scholarship at Oxford that he had won while he was still at school. On the same English literature course was Robert Graves, and the two were close friends during their time at Oxford together, but Blunden found university life unsatisfactory and left in 1920 to take up a literary career, at first acting as assistant to Middleton Murry on the Athenaeum.

An early supporter was Siegfried Sassoon, who became a lifelong friend. In 1920, Blunden published a collection of poems, The Waggoner, and with Alan Porter, he edited the poems of John Clare (mostly from Clare’s manuscript).

Blunden’s next book of poems, The Shepherd, published in 1922 won the Hawthornden Prize, but his poetry, though well reviewed, did not provide enough to live on. In 1924, he accepted the post of Professor of English at the University of Tokyo. In December 1925, he dedicated a poem « UP!UP! » to the rugby men of the University and this became the anthem of the Tokyo University RFC. He returned to England in 1927, and was literary editor of the Nation for a year. In 1927, he published a short book, On the Poems of Henry Vaughan, Characteristics and Intimations, with his principal Latin poems carefully translated into English verse (London: H. Cobden-Sanderson, 1927), expanding and revising an essay that he had published, in November 1926, in the London Mercury. In 1931, he returned to Oxford as a Fellow of Merton College, where he was highly regarded as a tutor. During his years in Oxford, Blunden published extensively: several collections of poetry including Choice or Chance (1934) and Shells by a Stream (1944), prose works on Charles Lamb; Edward Gibbon; Leigh Hunt; Percy Bysshe Shelley (Shelley: A Life Story); John Taylor; and Thomas Hardy; and a book about a game he loved, Cricket Country (1944). He returned to full-time writing in 1944, becoming assistant editor of The Times Literary Supplement. In 1947, he returned to Japan as a member of the British liaison mission in Tokyo. In 1953, after three years back in England he accepted the post of Professor of English Literature at the University of Hong Kong.

Blunden survived nearly two years in the front line without physical injury (despite being gassed in October 1917) but, for the rest of his life, he bore mental scars from his experiences. With characteristic self-deprecation he attributed his survival to his diminutive size, which made “an inconspicuous target”. His own account of his experiences was published in 1928, as Undertones of War.

Blunden retired in 1964 and settled in Suffolk. In 1966, he was nominated for the Oxford Professorship of Poetry in succession to Graves; with some misgivings, he agreed to stand and was elected by a large majority over the other candidate, Robert Lowell. However, he now found the strain of public lecturing too much for him, and after two years, he resigned.

He died of a heart attack at his home at Long Melford, Suffolk, in 1974, and is buried in the churchyard of Holy Trinity Church, Long Melford.

On 11 November 1985, Blunden was among 16 Great War poets commemorated on a slate stone unveiled in Poets’ Corner in Westminster Abbey. The inscription on the stone was written by fellow World War I poet Wilfred Owen: “My subject is War, and the pity of War. The Poetry is in the pity.”

Artists Rifles, an audiobook CD published in 2004, includes a reading of Concert Party, Busseboom by Blunden himself, recorded in 1964 by the British Council. Other World War I poets heard on the CD include Siegfried Sassoon, Edgell Rickword, Graves, David Jones and Lawrence Binyon. Blunden can also be heard on Memorial Tablet, an audiobook of readings by Sassoon issued in 2003.

In a 2009 appreciation of the book and its author, Bangalore writer Suresh Menon wrote: