Fr. John Grennon of the Assumption was born in Springfield, MA on August 31, 1946, to parents John and Kathleen (nee Peach) Grennon. He was professed as a Discalced Carmelite Friar on May 1, 1991, and made his solemn profession on April 27, 1996. On May 24, 1997, he was ordained to the priesthood at the Basilica of the National Shrine of Mary Help of Christians at Holy Hill in Hubertus, WI.

Before entering the order, Fr. John served in the United States Army (November 1970-May 1971) Fr. John served as a Secular Order Delegate for the Washington Province. He was assigned to Holy Hill to serve with the Hispanic ministry, at Holy Espousal Monastery in Brighton, MA, and the Sacred Heart Monastery (St. Florian Parish in Milwaukee, WI). He ministered to the residents at Mitchell Manor and Sacred Heart Care Center of the School Sisters of St. Francis.

Fr. John died on June 29, 2019, and is buried at the Carmelite Cemetery in Hubertus, Wisconsin.