He revived the interest in Celtic spirituality and Druidry in the 20th century. Nichols was a Member and Chairman of the Ancient Druid Order which traces its lineage to a meeting at the Apple Tree Tavern in Covent Garden, London, in 1717. His main work, The Book of Druidry, was published posthumously in 1990.
Nichols was a friend of Gerald Gardner, and while Gardner worked to introduce Wicca to the modern world, Nichols worked to change the practice of modern Druidry. He introduced a concern for Celtic mythology and Bardcraft, and the celebration of the full eight seasonal ceremonies in addition to arranging the teachings into three grades, in accordance with classical accounts of the three divisions of the Druids.
Nichols also translated Jean Baptiste's (aka Paul Christian) book "The History and Practice of Magic" in 1969.