Stephen was the second son of Sir James Fitzjames Stephen, barrister-at-law, and his wife Mary Richenda Cunningham. James Kenneth Stephen was known as 'Jem' among his family and close friends; he was first cousin to Virginia Woolf (née Stephen).
He was a King's Scholar at Eton, where he proved to be a highly competent player of the Eton Wall Game; and then went up to King's College, Cambridge, again as a King's Scholar. In the Michaelmas term of 1880, he was President of the Cambridge Union Society. In 1883 he became tutor to Prince Albert Victor, Duke of Clarence and Avondale, and was made a Fellow of King's College in 1885. He was a renowned intellectual; and it was said that he spoke in a pedantic, but highly articulate and entertaining manner.
Stephen became a published poet, his work being identified by the initials J. K. S. His collections of poems Lapsus Calami and Quo Musa Tendis were both published in 1891. Rudyard Kipling called him "that genius" and told how he "dealt with Haggard and me in some stanzas which I would have given much to have written myself". Those stanzas, in which Stephen deplores the state of contemporary writing, appear in his poem 'To R. K.':
"The Last Ride Together (From Her Point of View)" parodies Robert Browning's "Last Ride Together"; Lord Byron is parodied in "A Grievance"; and William Wordsworth in "A Sonnet":
J. K Stephen was at Cambridge at the same time as the distinguished antiquarian and writer of ghost-stories, Montague R. James, and mentions him at the end of a curious Latin celebration of then-current worthies of 'Coll. Regale' (King's College):
Stephen wrote a satirical pastiche of Thomas Gray's "Ode to the Distant Prospect of Eton College" pillorying Eton for being Tory.
A poem which gave him a reputation as a misogynist is "Men and Women," where he describes two people, a man and a woman, whom he does not know but to whom he takes a violent dislike. The first part, subtitled "In the Backs" (The Backs is a riverside area of Cambridge), concludes
(Plough is slang for failing an exam.)
However many of his other poems show that this "misogyny" Is more accurately described as only one facet of a sardonic nature.
Stephen was a member of the Cambridge "Apostles".
Stephen suffered a serious head injury in an accident in the winter of 1886/1887 which may have exacerbated the bi-polar disorder from which he suffered. His cousin Virginia Woolf suffered from the same disorder throughout her adult life. Stephen was eventually committed to St Andrew's Hospital, a mental asylum in Northampton.
In January 1892 the former Royal tutor heard that his erstwhile pupil, the 28-year-old Prince Albert Victor, Duke of Clarence, a man destined to one day be King, had died of pneumonia at Sandringham, after contracting influenza. On hearing the news, Stephen refused to eat, and died twenty days later, aged 32. His cause of death, according to the death certificate, was mania.
Stephen was noted for his prodigious size and physical strength. At Eton, he was an outstanding player of the Wall Game. He played for College on St Andrew's Day four times: in 1874, 1875, 1876 and 1877. In the last two years he was Keeper (or captain) of the College Wall. College beat the Oppidans by 4 shies to nil in his first year as Keeper, and by 10 shies to nil the next year. Ever after, the King's Scholars have honoured J K Stephen's memory with a toast at the Christmas Sock Supper or other festive occasions - in piam memoriam, J. K. S. (In pious memory of J. K. S.).
Stephen was recalled in less pious memory in a play by former Eton housemaster and Old Etonian, Angus Graham-Campbell; entitled Sympathy for the Devil, it premiered at the Eton Drama festival in 1993. This was based on the notion that Stephen could have been one of the Jack the Ripper suspects; this theory has been dismissed, because he would have been unable to return to Cambridge in time for lectures the following morning.
Stephen's poem The Old School List from Quo Musa Tendis is included in the front pages of H. E. C. Stapleton's Eton School Lists 1853-1892, and the author refers to him in the preface as 'an Etonian of great promise, who died only too early for his numerous friends'. During his time at Eton, Stephen was a friend of Harry Goodhart (1858–1895), who became an England international footballer and later a Professor at the University of Edinburgh. Goodhart is referred to as "one of them's wed" in the last verse of The Old School List:Select Poems 1926 Augustan Books of Modern Poetry
Lapsus Calami JKS Cambridge 1891
Quo Musa Tendis Cambridge 1891
Lapsus Calami and other verses 1896