Farrar was born in Clapham in 1950. Her family, of mixed English, Irish and Welsh descent, were members of the Church of England. Farrar attended the Leyton Manor School, and the Royal Wanstead High School girls' school. After high school, Farrar worked as a model and receptionist.

Farrar was initiated into Alexandrian Wicca by the tradition's founders, Alex and Maxine Sanders. Farrar met the Sanders in 1970 through a friend who had become interested in exploring Wicca. Farrar accompanied her friend in order to keep the friend "out of this weird cult", but Farrar instead joined the Sanders coven, and would go on to become, in the words of Knowles, one of "England’s most eminent and respected modern day witches." In the coven she met Stewart Farrar, her future husband and co-author.

Janet Farrar asserts that the couple were both elevated to the second degree "in an unoccupied house in Sydenham" by the Sanders on 17 October 1970, and that they received the third, and final, degree of initiation in their flat on 24 April 1971, but that these events are disputed by some Alexandrian "revisionists".

The Farrars had begun running their own coven in 1971, before their third degree initiation ceremony, and were handfasted in 1972 and legally married in 1975. Janet Farrar left the coven in 1972 to explore Kabbala with a ceremonial magic lodge, but returned within the same year. In 1976 the Farrars moved to Ireland to get away from the busy life of London. They lived in County Mayo and County Wicklow, finally settling in "Herne Cottage" in Kells, County Meath. Both husband and wife went on to publish a number of books on the Wiccan religion and on coven practises. Farrar continued to model and appeared in the illustrations to multiple early books about Wicca, including the cover of the paperback version of Margot Adler's 1979 Drawing Down the Moon. Farrar also posed for many of the photographs in their 1981 Eight Sabbats for Witches, which included material the authors claimed to be from the Alexandrian tradition's Book of Shadows. The Farrars, with the support of Doreen Valiente, argued in the book that even though the publishing of this material broke their oath of secrecy, it was justified by the need to correct misinformation. Janet Farrar indicates that some of the rituals contained in the couple's books were actually written by them, and that they left the Alexandrian tradition after the book's research was complete. The couple co-authored four more books on Wicca. Janet Farrar's post-Alexandrian practice has been referred to as "Reformed Alexandrian".

The Farrars returned to England in 1988, but by 1993 had returned to Ireland. They were joined by Gavin Bone, with whom they entered into a "polyfidelitous relationship". The three of them would co-author two more books, The Healing Craft and The Pagan Path, an investigation into the many varieties of Neopaganism. Stewart Farrar died in February 2000 after a brief illness.

After Stewart Farrar's death, Janet Farrar and Gavin Bone have continued to author books, and have given a number of lectures on Wicca in the United States, Australia and in Britain. The title of their 2004 book, Progressive Witchcraft, is the description that the couple prefers for their current religious practice.

Farrar has co-authored a number of books about Wicca and Neopaganism.

With Stewart Farrar

• 1981: Eight Sabbats for Witches
• 1984: The Witches' Way
• 1987: The Witches' Goddess: The Feminine Principle of Divinity
• 1989: The Witches' God: Lord of the Dance
• 1990: Spells and How they Work
• 1996: A Witches' Bible: The Complete Witches' Handbook (re-issue of The Witches' Way and Eight Sabbats for Witches)

With Stewart Farrar and Gavin Bone

• 1995: The Pagan Path
• 1999: The Healing Craft: Healing Practices for Witches and Pagans
• 2001: The Complete Dictionary of European Gods and Goddesses

With Virginia Russell

• 1999: The Magical History of the Horse

With Gavin Bone

• 2004: Progressive Witchcraft: Spirituality, Mysteries, and Training in Modern Wicca

The wiccan ways- Applionicisiumn by : Rebecca M. Lee