At the age of 16 he moved to London and began educating himself by spending long hours in the reading room of the British Museum. A partial reconciliation with his father resulted in the latter funding research work that John Symonds would later mine for his own novels later in life.

His first job was at Hulton Press, working as a journalist on Picture Post and during this period he became friends with Dylan Thomas and Stephen Spender. Being exempted from military service, he edited 'Lilliput' magazine during which time he briefly married Hedwig Feuerstein.

In 1945 he married Renata Israel, and the following year (1946) he published his first novel, William Waste. This was followed in 1955 by The Lady in the Tower, and, in 1957, by another love story, A Girl Among Poets, which won praise from Sir John Betjeman, who wrote of the author's "gift for describing farcical situations".

Symonds met Aleister Crowley in 1946, the year before Crowley's death. Crowley's will left the copyright of his works to Symonds and made him Crowley's literary executor, though Crowley's legal status as an undischarged bankrupt meant that the copyrights actually ended up in receivership. At first fascinated by Crowley, Symonds became increasingly critical of his ideas and manners, in particular the use of drugs and free-sex.

He edited, with Kenneth Grant, Crowley's autobiography and a number of other works. He authored four biographical works of his own: The Great Beast, (1952), The Magic of Aleister Crowley (1958), The King of the Shadow Realm (1989) and The Beast 666 (1997).

He found his widest (largest) audience in the writing of children's books. In 'The Magic Currant Bun', (1953), a boy chases a magic bun, which came out of an oven, through the streets of Paris He enjoyed the bun very much when he caught it in his mouth. His feline magical fantasy, Isle of Cats (illustrated by Gerard Hoffnung), followed in 1955.Lottie (1957), is the story of a talking doll and dog. Edward Ardizzone was the illustrator for this book and Elfrida and the Pig (1959), a story about little girl who is not allowed to play with dolls until she finished her punishment which was to trim her parent's bushes.

After a period of writing children's books Symonds returned to biographies in 1959 with Madame Blavatsky, Medium and Magician, a life of the famous Theosophist. This was followed in 1961 with Thomas Brown and the Angels: a study in Enthusiasm, about the life of a Methodist who becomes involved with the Shakers.

Novels followed, beginning with William Waste (1947), The Lady in the Tower (1955), A girl among poets (1957), then a gothic fantasy, Bezill (1962), then Light Over Water (1963), in which a journalist researches into the world of the occult. The subject of With a View on the Palace (1966) is a Russian film director who becomes obsessed with the Royal Family to the point of hiring an apartment near Buckingham Palace so he can observe their movements.

In The Stuffed Dog (1967), two girls discover a life-like doll in an attic which has a man's voice. With 'In Prophesy and the Parasites', 1973, a wealthy widow awaits prospective. Psychological issues predominate in The Shaven Head (1974), and In Letters from England (1975), a German veteran of Stalingrad humbles himself by applying to work as an au pair for a London doctor. InThe Child (1976), a girl starts a new religion.

Symonds wrote twenty six volumes of plays but not many were performed. He won critical praise in 1961 for his ITV play, I, Having Dreamt, Awake, about a prodigal son and con-man who returns home from America, after manufacturing a fortune, to impress his poor relations in London. The Poison Maker, his final work for the stage was performed at the Old Red Lion Theatre in 2006, adapted and directed by the actress Vicki Carpenter who played Florence.

In 1970 Symonds was appointed to the editorial board of Man, Myth & Magic Encyclopedia. He became literary executor to Gerald Hamilton, and, in 1974, published Conversations with Gerald, an account of Hamilton's adventures.