Born in Punxsutawney, Pennsylvania in 1823, Barclay attended Washington College (now Washington & Jefferson College) in Washington, Pennsylvania. He studied law in Pittsburgh, was admitted to the bar and practiced in Punxsutawney, Brookville, and Kittanning, Pennsylvania."Barclay, David" (B000136). in Biographical Directory of the United States Congress."Barclay, David," The Political Graveyard.
