Grimassi is the son of an Italian immigrant who was born and raised in the area of Naples, Italy. His mother came to the U.S. as a war bride at the close of World War II. Being raised by a European mother helped Grimassi forge a strong sense of self-identity as he was, in effect, raised in two different worlds. Growing up as an Italian-American in many ways caused a separation from other people and created social challenges for Grimassi. His family moved frequently from state to state because of his father's career, which continually exposed Grimassi to new situations and environments.

Grimassi is reportedly descended from an Italian witch named Calenda Tavani, who lived in Naples several generations ago. Grimassi states that his early training was a mixture of Italian witchcraft and folk magic.His later interest in Neo-paganism began in 1969, and he was initiated into a system claiming to be Gardnerian Wicca in San Diego (the tradition's claim eventually proved to be false). Ten years later, Grimassi began teaching the "Aridian Tradition", which he describes as a "modern system" of Italian Witchcraft (Stregheria) that he created for non-initiates. According to Grimassi, one of his students was Scott Cunningham. Grimassi also studied Kabbalah and other traditions of Wicca such as Brittic and the Pictish-Gaelic system in which he received third degree initiation in 1983 (Encyclopedia of Wicca & Witchcraft, Llewellyn Publications, 2003). Grimassi also received third degree initiation into Traditionalist Celtic Wicca in 2001 (First Wiccan Church of Escondido, California).

In 1994, the new age publisher Llewellyn Publications accepted his manuscript for Ways of the Strega, which was reprinted the following year as Italian Witchcraft: The Old Religion of Southern Europe . The publication of this book launched Grimassi's professional career as an author. Grimassi went on to produce over a dozen popular books on Witchcraft and Wicca.

He currently lives in Massachusetts and is the directing elder of Arician Witchcraft and co-director of the College of the Crossroads. Grimassi was formerly the editor of the magazine Raven's Call.

Grimassi places pride in his Italian-American heritage, and has written two books that present a modernized public version of an Italian Witchcraft system based upon an old tradition that was earlier taught to him. Grimassi points out that one does not have to be of Italian descent in order to practice Italian Witchcraft

Grimassi differs from his contemporaries at Llewellyn, such as Cunningham and Silver RavenWolf, in offering footnotes and references in his books, and by suggesting academic texts and older classic works (such as The Golden Bough) for further reading. His publisher, in the "About the Author" section of his books Italian Witchcraft, and Hereditary Witchcraft, describes him as "a recognized expert on Italian Witchcraft and the foremost authority on the works of Charles Godfrey Leland in this particular field". Grimassi displays this text on his personal author website as well.

Grimassi's intimation of belonging to a "family tradition" of religious witchcraft, a claim made by a number of other Llewellyn authors and which has never been substantiated, has opened him to criticism. Grimassi responds by saying that, although he wrote about such a family tradition, he never specifically mentions his own family, and it was only Llewellyn's marketing department that made such a statement. Professor Sabina Magliocco, who has criticized some of Grimassi's claims, does point out that "Grimassi never claims to be reproducing exactly what was practiced by Italian immigrants to North America; he admits Italian-American immigrants "have adapted a few Wiccan elements into their ways". After personally meeting Grimassi, Professor Magliocco writes in her letter to the Pomegranate Reader's Forum: "I had the pleasure of meeting Raven Grimassi during the summer of 2001, unfortunately after the final draft of my article had already been submitted to The Pom. He was very gracious and helpful to me. From information he revealed during our interview, I can say with reasonable certainty that I believe him to have been initiated into a domestic tradition of folk magic and healing such as I describe in my article." As such a tradition-bearer, Grimassi's writings are a unique and valuable contribution to the contemporary writings on Wicca and Witchcraft.

Grimassi is well known for his research on Charles Leland and Italian witchcraft. He has studied these subjects for over thirty years. Grimassi views Leland's book Aradia, or the Gospel of the Witches as a "Christianized and distorted version" of the original story of Aradia, whom he believes to be a mortal woman named Aradia di Toscano. However, Grimassi does point to genuine elements within Leland's Aradia material, such as the inclusion of a full moon ritual and a sacred meal at the Tregenda (Sabbat), along with the pantheon of a goddess and god figure. It is only Leland's particular depiction of these aspects that Grimassi feels are distortions.

At the Pantheacon conference, in San Jose, California, on February 17, 2008, Grimassi presented new and groundbreaking information related to Charles Leland and his witch-informant, Maddalena. One of the documents was a copy from The International Folklore Congress: Papers and Transactions, 1891. The page showed a list of items on exhibit by Charles Leland with a credit to "Maddalena Taluti" as having provided a particular item. Grimassi also produced another document revealing that Maddalena's home town was Rocca Casciano, which is located just a few miles outside of Florence. In the 1998 publication of the Pazzaglini translation of Aradia, contributing writer professor Robert Mathiesen gave Maddalena's last name as Talenti, which does not match her name in the Folklore publication of 1891. However, Mathiesen does state that on the document he examined, Maddalena's name is difficult to make out because of the handwriting.

As indicated in his writings, Grimassi considers it his life work to preserve and present sources of information related to his belief in the survival and continuation of witchcraft as a religion and practice of great antiquity.