His father was the biologist Thomas Henry Huxley. He was educated at University College School, London, St. Andrews University, and Balliol College, Oxford. He married firstly Julia Arnold, daughter of Tom Arnold and a sister of the novelist Mrs. Humphrey Ward, niece of the poet Matthew Arnold, and granddaughter of Dr. Thomas Arnold, the headmaster of Rugby School who was immortalised as a character in the novel Tom Brown's Schooldays.
Their four children included the biologist Sir Julian Sorell Huxley (born 1887) and the writer Aldous Leonard Huxley (born 1894). His first wife died of cancer in 1908 and in the same month his daughter Margaret (born 1899) passed away. Their middle son, Trevenen (born in 1889) committed suicide in 1914.
After the death of his first wife, Leonard married Rosalind Bruce (said to be related in some way to Robert the Bruce), and had two further sons. The elder of these was David Bruce Huxley (born 1915), whose daughter Angela married George Pember Darwin, son of the physicist Sir Charles Galton Darwin (and thus a great-grandson of Charles Darwin married a great-granddaughter of Thomas Huxley) and the younger was Nobel Prize-winning physiologist Andrew Fielding Huxley (born 1917).
Huxley edited three volumes of Life and Letters of Thomas Henry Huxley and also published Thomas Henry Huxley A Character Sketch. He was assistant master at Charterhouse School between 1884 and 1901. He was then the assistant editor of Cornhill Magazine between 1901 and 1916, becoming its editor in 1916.