He was the son of a merchant who ran a family firm of wine and spirit importers, Matthew Clark & Sons. The socialist politician Susan Lawrence was his aunt. He was educated at St Aubyn's, Rottingdean, and at Rugby. He studied at New College, Oxford (where he heard William Archibald Spooner say in a sermon that 'now we see through a dark glassly'), from where he graduated with a First in History. 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 Ronald 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 Alexandra Wedgwood, architectural historian) and Cecilia Mary Gordon Clark (Cecilia Snell, musician).
As a young man and during the early days of World War II 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. Needless to say, this slightly crabby barrister-hero was a Temple man. His other detective (they appeared together in three novels) was a very large 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.