The second Viscount Runciman of Doxford, elder son of the cabinet minister, was born in Newcastle and grew up at Doxford Hall. He was educated at Eton and Cambridge and then entered the family shipping business, learning the ropes from his formidable grandfather (see above). In time, Leslie became chairman of the company as well as six other banking and shipping concerns. He later held many offices including the presidency of the Royal Institution of Naval Architects (1951-61). A keen sailor himself, he went cruising in the family's three-masted schooner Sunbeam and later in his motor ketch Bondicar. He succeeded the Duke of Edinburgh as Commodore of the Royal Yacht Squadron (1968-74).
In the 1920s, Runciman qualified as a pilot, taking part in the King's Cup air races and founding his own aviation company. He commanded the Durham Squadron of the Auxiliary Air Force and was awarded the Air Force Cross in 1937. He played a prominent part in amalgamating Imperial Airways and British Airways into what became BOAC, of which Runciman was the first director-general. After the death of his father, Runciman became absorbed in the shipping affairs of the Runciman companies which included the Moor Line, the Doxford Co. Ltd. and the Anchor Line. He also served for forty years on the board of Lloyds Bank.
Runciman was tall and handsome, with great personal charm. Surprisingly for a man of affairs, he was also an intellectual, an omnivorous reader and president of the Horatian Society (1970-88). He was also fond of opera. He married the writer Rosamond Lehmann in 1923, and the couple lived at 72 Sydenham Terrace (now demolished) in Newcastle. The union did not prosper, however, and was eventually dissolved in 1928. Rosamond wrote of this unhappy period in her second novel A Note in Music.