Frank Stewart Farrar (28 June 1916 – 7 February 2000) was an English screenwriter, novelist and prominent figure in the Neopagan religion of Wicca, which he devoted much of his later life to propagating with the aid of his seventh wife, Janet Farrar, and then his friend Gavin Bone as well. A devout communist in early life, he worked as a reporter for such newspapers as the Soviet Weekly and the Daily Worker, and also served in the British army during the Second World War. He was responsible for writing episodes for such television series as Dr. Finlay's Casebook, Armchair Theatre and Crossroads, and for his work in writing radio scripts won a Writer's Guild Award. He also published a string of novels, written in such disparate genres as crime, romance and fantasy.

After being initiated into Alexandrian Wicca by Maxine Sanders in 1970, he subsequently published one of the earliest books to describe this newly burgeoning religion, What Witches Do (1971). Within only a few months of being initiated, he had risen to the position of High Priest and founded his own coven in south London, with Janet Farrar, whom he would later handfast and then legally marry, as his High Priestess. In 1976 the couple moved to Ireland, where they went about founding new covens and initiating new people into Wicca - according to George Knowles, "some seventy five per cent of Wiccans both in the Republic and North of Ireland can trace their roots back to the Farrar's [sic]". With Janet, he also set about writing books about the subject, most notably Eight Sabbats for Witches (1981) and The Witches' Way (1984).

Because of his work in propagating the Craft, the historian Ronald Hutton compared him to Gerald Gardner and Alex Sanders as "the third and last of the great male figures who have formed Wicca".

Stewart Farrar was born at his family home of 239 Winchester Road, Highams Park, Essex during the First World War, and as such his father was away serving in the British army, stationed in Salonika in Greece. His family were middle class and well educated, and were also Christian Scientists, a denomination of Christianity that notably emphasised a belief in spiritual healing over conventional medicine, and which had been founded in 1886. The Farrar family had already been somewhat successful, with a number of them becoming somewhat culturally significant: the first words that had been broadcast by radio across the Atlantic, Guglielmo Marconi's "can you hear me, Picken?", for instance referred to Stewart's maternal uncle, whilst Stewart's first cousin was the notable poet James Farrar.

His father, after being demobbed from the army, took up employment at the London office of the Hong Kong and Shanghai Bank, whilst his mother ran a private school in Wallington, Surrey, before later teaching at the Christian Science primary Claremont School in Esher, Surrey, where the Farrar family took up residence in a house on the school's grounds. His sister, Jean, was born in 1920, and he subsequently doted on her, but at the same time was known as a bully towards other children at primary school.

From 1930 to 1935, Stewart attended the privately run City of London School, meaning that he had to commute daily into the city from his rural home. It was here that he joined the Officer Training Corps, where he learned much about military strategy, but at the same time disapproved of militarism and began to sympathise with left wing politics that were at odds with his conservative upbringing. At 17, he became a socialist, before later taking a further leftist stance by declaring himself a communist and rejecting Christianity, instead defining himself as an "interested agnostic". In 1935 he began attending University College London, where he studied journalism, and where he served both as president of the London University Journalism Union and editor of the London Union Magazine. After he ended his university education in 1937, he spent three months as an exchange student in Dresden, Germany, where he became fluent in German and also developed an even greater hatred of National Socialism than he already held.

Returning to Britain, Farrar immersed himself in propagating communism, joining the Communist Party of Great Britain, and working for the communist tabloid, the Daily Worker. In 1939, he fell in love with and married Jean Clarke, a fellow communist.

When war broke out against Nazi Germany in 1939, he immediately volunteered for the British army, feeling that he could put his military training to good use to fight against fascism. Instead of being sent abroad to fight, he was stationed in Britain, where he was set to work training new recruits in various combat techniques, and as such was stationed at a variety of different barracks. In 1940 he was stationed at Whitstable, though soon moved to Ashford, and the following year, his first son, Tony, was born. He was subsequently moved to Pembrokeshire in Wales, where he began having an affair with his secretary. Soon after this he divorced his wife, who was also having an affair. Meanwhile, his war work largely involved working as an instructor in Anti-aircraft gunnery during World War II. He also wrote an instruction manual for a Bofors gun.