Born Joseph Grennan in New York on May 25, 1869 he was first introduced to the Passionists by serving as a door boy at St. Michael’s Monastery in West Hoboken, New Jersey. At 16 Grennan entered the Passionist novitiate at St. Paul of the Cross Monastery, Pittsburgh, Pennsylvania.. With the name Stanislaus he professed his vows on October 31, 1886. On May 27, 1893 Grennan was ordained a priest by Bishop Winand Wigger of Newark.
Father Grennan was taught Sacred Eloquence (preaching techniques) by Father Fidelis Kent Sone, C.P. Next he was Vice-Master of Novice for two years. Later he was Director of Students in which he also served as a Lector. He was Rector at Holy Cross Monastery in Cincinnati, Ohio. Then he was Master of Novices and then six years as Provincial. As First Consultor to the Provincial Father Grennan also taught Sacred Eloquence and Moral Theology. He then served another term as Provincial after which he was appointed Rector of the Preparatory Seminary at Dunkirk, New York until a serious health breakdown necessitated he resign. Recovered, he once again began to teach Sacred Eloquence.
Father Grennan was a superb and inspiring preacher and instilled a strong devotion to the Blessed Mother, the novenas, and the Passionist saints. His educational efforts included a restructuring of the seminary system and support of Sign Magazine . In addition he published encyclicals on Modernism, and the Regular Observance. He was committed to the expansion of the Passionists and went to Rome and successfully advanced the cause that the Passionists go to China and Germany. During the Grennan years of leadership he invited the Passionist Nuns to the United States, a site was chosen at Shelter Island, New York, the Preparatory Seminary at Dunkirk was built as were monasteries in Brighton, Massachusetts, West Springfield, Massachusetts, and Jamaica, New York. New monastery churches were built at St. Ann’s Scranton, Pennsylvania. Property was also acquired at Riverdale, New York.