Born at Opotiki in 1887, he was the eighth and youngest child of Alfred John Sisam, a police officer and farmer, and his wife Maria Knights. He was educated at Auckland Grammar School, and entered University College, Auckland, in 1906 with a scholarship, where he graduated MA in 1910.
With a Rhodes scholarship, Sisam matriculated at Merton College, Oxford, in 1910. He completed a B.Litt. there under Arthur Napier in 1915, producing an edition of the Salisbury Psalter. He married that year. In this period he taught students including J. R. R. Tolkien. Poor health ruled out military service, and he went to work part-time on the Oxford English Dictionary. In 1916 he published on the Beowulf manuscript.
In 1917 the Sisams moved to London, where Kenneth worked as a civil servant. In 1922 he joined Oxford University Press (OUP). With his promotion to assistant secretary, they built a family house at Boars Hill. From 1922 to 1942 Sisam worked at OUP under Robert William Chapman while developing his scholarly work on Anglo-Saxon, but failing in 1925 to become Rawlinson and Bosworth Professor of Anglo-Saxon when Tolkien was chosen. OUP successes under his stewardship include introducing 30 new titles to the Oxford World's Classics series; the creation of the Oxford Companion to English and the Oxford Latin Dictionary, and the recruitment of W. B. Yeats as editor of The Oxford Book of Modern Verse.
Sisam was elected to the British Academy in 1941. Appointed OUP secretary in succession to Chapman in 1942, he became a Fellow of Merton College. In 1948 he retired to the Scilly Isles but continued to produce scholarship, including The Structure of Beowulf (1965). He died in a nursing home at Lelant in Cornwall on 26 August 1971.
- Stray, Christopher. "Sisam, Kenneth". Oxford Dictionary of National Biography (online ed.). Oxford University Press. doi:10.1093/ref:odnb/94507. (Subscription or UK public library membership required.)
- Levens, R.G.C., ed. (1964). Merton College Register 1900–1964. Oxford: Basil Blackwell. p. 81.
- John Garth, Tolkien and the Great War: The Threshold of Middle-earth (Boston: Houghton Mifflin, 2003), p. 34.
- John M. Bowers, Tolkien's Lost Chaucer (Oxford: Oxford UP, 2019), p. 56.