William Gannon was born on March 20, 1858, in Cambridge, Massachusetts to parents Patrick and Martha (Whalley) Gannon. He studied at Boston College High School and entered the Society of Jesus on August 5, 1876. After two years as a novice and two years in his classical studies, he went to Woodstock College to study philosophy. From 1883 to 1885, Gannon taught at the College of the Holy Cross in Worcester, Massachusetts.
In 1888, Gannon returned to Woodstock College for his theological studies, and he was ordained a priest in June 1891. In 1894, Gannon became the prefect of discipline at Georgetown University. On July 30, 1903, Gannon succeeded W. G. Read Mullan as the president of Boston College. Gannon sought to expand the college's sports programs, and its intramural baseball team was re-established. Gannon was succeeded as president of Boston College by Thomas I. Gasson on January 6, 1907. On October 30, 1916, Gannon suffered a stroke and died in Philadelphia, Pennsylvania.
