Born in Aylmer, Lower Canada, the son of Charles Devlin and Ellen Roney, his father was a merchant from Roscommon in Ireland.

After attending the Petit Séminaire de Montréal from 1871 to 1877, he studied at the Université Laval in the faculty of arts from 1879 to 1881 but did not graduate.

It is uncertain what his profession was before being elected as the Liberal candidate in 1891 to the House of Commons of Canada for the riding of Ottawa (County of) with the help of his friend Henri Bourassa.

In 1902, against the advice of Canadian Prime Minister Wilfrid Laurier, Devlin was elected by acclamation to the British House of Commons as the Irish Parliamentary Party candidate for Galway Borough.

He resigned in 1907, after being appointed minister of colonization, mines, and fisheries in the cabinet of Quebec Premier Lomer Gouin.