A descendant of the Ó Siadhail family, Shields was born in Banbridge, County Down in the Kingdom of Ireland.
He received a good common-school education, entering the University of Glasgow, Scotland in 1782 and graduated in 1786.
He attended medical college for two years, before emigrating to the United States in July 1791 and settled in Frederick County, Virginia, where he taught school.
He moved to Butler County, Ohio in 1801, but then returned to Virginia and became a citizen of the United States in 1804.
Shields was killed through the accidental overturning of a stagecoach near Venice, Butler County, Ohio on August 13, 1831.