Born in Scotland, United Kingdom in 1824, Gilmour moved to Nova Scotia, Canada, then Pennsylvania, USA as a child. Raised Presbyterian, he converted to Catholicism around age 20 and entered the priesthood. In 1852, he was ordained a priest and, in 1872, he was ordained bishop of Cleveland, serving until his death in 1891 at age 66. Gilmour was a strong proponent of parochial education. He also was instrumental in the opening of a Catholic University in Cleveland, Ohio (St. Ignatius College, now John Carroll University).