Cecil Harmsworth King was born on 20 February 1901 at Poynters Hall, Totteridge, Hertfordshire, the home of his grandmother, Geraldine Mary Harmsworth. He came on his father's side from a Protestant Irish family, and was brought up in Ireland. His father was Sir Lucas White King, Professor of Oriental Languages at Trinity College, Dublin and his mother was Geraldine Adelaide Hamilton (née Harmsworth), daughter of Alfred Harmsworth, a barrister, and sister of the mass-circulation newspaper proprietors Alfred Harmsworth, 1st Viscount Northcliffe and Harold Sidney Harmsworth, 1st Viscount Rothermere.
The fourth child in the family of three sons and three daughters, he was educated at Winchester College and Christ Church, Oxford. According to Geoffrey Goodman: "He believed he was born to rule, an image of himself which never departed."
In 1937, he was an advertising director of one of his uncle's papers when he formed a partnership with journalist Hugh Cudlipp. When he was made a senior director, he chose Cudlipp as his new editor. At the age of 23, Cudlipp became the youngest chief editor in Fleet Street. Between them, both men turned The Daily Mirror into the world's largest selling daily newspaper. In 1967, the Daily Mirror's circulation reached a world record of 5,282,137 copies. By 1963, King was chairman of the International Publishing Corporation (IPC), then the biggest publishing empire in the world, which included the Daily Mirror and some two hundred other papers and magazines (1963-1968). His influence in British public life was enormous. He himself believed that criticism of Winston Churchill's government by the Mirror had caused that government's collapse after the war.
King was involved in, and probably instigated, a bizarre 1968 meeting with Louis Mountbatten, among others, in which he proposed that Harold Wilson's government be overthrown and replaced with a temporary administration headed by Mountbatten. He had no support from them for this, so he decided to override the editorial independence of the Mirror and wrote and instructed to be published a front page article calling on Wilson to be removed by some sort of extra-parliamentary action. The board of IPC met and demanded his resignation for this breach of procedure and damaging the interests of IPC as a public company. He refused, so was dismissed by the board on 30 May.
He married, firstly, Agnes Margaret Cooke, daughter of Canon George Albert Cooke and Frances Helen Anderson, in 1923. They had four children: Michael, Francis, Priscilla and Colin. He and Agnes Margaret Cooke were divorced. He married, secondly, Dame Ruth Railton in 1962, founder of the National Youth Orchestra of Great Britain, daughter of Rev David Railton and Ruby Marion Wilson.
In 1974, he moved from London to Dublin with his second wife. He died at his Dublin home, The Pavilion, 23 Greenfield Park, following a long illness. He was survived by Dame Ruth as well as his children by his first marriage.