Hare was the third son of Henry Herbert Gordon Clark of Mickleham Hall, a merchant in the wine and spirit trade, Matthew Clark & Sons being the family firm. The socialist politician Susan Lawrence was his aunt. He was educated at St Aubyn's, Rottingdean and Rugby. He read History at New College, Oxford (where he heard William Archibald Spooner say in a sermon that 'now we see through a dark glassly') and graduated with a First. Then he studied law and was called to the Bar (Middle Temple) in 1924.
Hare's pseudonym is a mixture of Hare Court, where he worked in the chambers of Roland Oliver, and Cyril Mansions, Battersea, where he lived after marrying Mary Barbara Lawrence (see Lawrence Baronets, Ealing Park) in 1933. They had one son, Charles Gordon Clark (clergyman, later dry stone waller), and two daughters, Alexandra Mary Gordon Clark (Lady Wedgwood, architectural historian, see Wedgwood Baronets) and Cecilia Mary Gordon Clark (Cecilia Snell, musician).
As a young man and during the early days of World War II Gordon Clark toured as a judge's marshal, an experience he used in Tragedy at Law. Between 1942 and 1945 he worked at the office of the Director of Public Prosecutions. At the beginning of the war he served a short time at the Ministry of Economic Warfare, and the wartime civil service with many temporary members appears in With a Bare Bodkin. In 1950 he was appointed county court judge in Surrey. His best known novel is Tragedy at Law, in which he drew on his legal expertise and in which he introduced Francis Pettigrew, a not very successful barrister who in this and four other novels just happens to elucidate aspects of the crime. His professional detective (they appeared together in three novels, and only one has neither of them present) was a large and realistic police officer, Inspector Mallett, with a vast appetite. Among the more outstanding of Hare's literary contributions are his short stories, mostly written for the London Evening Standard.