In 1913 he returned to the stage touring as the Rt Hon. Denzil Trevena in his own play, "The Earth" (originally produced in 1909). He next wrote "The Fourth of August" (1914) and "Doctor O'Toole" (1917). In 1917 he produced his first play, his own adaptation of the Brieux play "Damaged Goods" at St Martin's Theatre. He next produced "The Wonder Tales" and "The Little Brother" at the Ambassadors' Theatre in London. He took over the Court Theatre in London's Sloane Square as a Shakespearean playhouse in 1920. "The Times" called his revivals of "Twelfth Night", "The Merchant of Venice", "Henry the Fourth (Part Two)" and "A Midsummer Night's Dream" "memorable for their freshness, sanity and distinction, and [deserving of] a place in theatrical history". "The Merchant of Venice" transferred to the Duke of York's Theatre, where Fagan also produced "The Government Inspector" and "Madame Sand" (both 1920). At the Court, he revived "Damaged Goods" and, in 1921, with the assistance of the author, produced G. B. Shaw's "Heartbreak House", with Edith Evans as "Lady Utterwood". This was not a success and folded after 63 performances. In 1922 he produced his play "The Wheel" at the Apollo Theatre. Its success allowed him to repay his creditors. Even more successful was his adaptation of "Treasure Island" at the Savoy Theatre with Arthur Bourchier as "Long John Silver", which opened 26 December 1922. It was to be revived every Christmas until the outbreak of World War II. Fagan was persuaded by Jane Ellis, the actress who with Alfred Ballard founded the Oxford Playhouse "Red Barn" in 1923, to be its first manager. A misfortune occurred while his effects were being transferred from London to Oxford; the lorry caught fire at Gerrard's Cross, and his rare book collection and irreplaceable original writings were destroyed, as well as stage properties and costumes. His attempt to license the theatre was stymied by the University's Vice-Chancellor, Dr Lewis Farnell, who had the power to prohibit staging of plays of which he disapproved (he had banned a Grand Guignol play starring Sybil Thorndike in Oxford in 1922 and a lecture by birth-control pioneer Marie Stopes in 1923). But Fagan's supporters, including the Chancellor George Curzon, forced a partial backdown.