In 1857, he became registrar of deeds and wills for Victoria County in New Brunswick and a judge of the Inferior Court of Common Pleas.

He resigned from these posts in 1861 when he was elected to be a member of the 19th New Brunswick Legislative Assembly, representing the electoral district of Victoria.

[1][2] On September 20, 1867, he was elected to the 1st Canadian Parliament as a member of the Liberal-Conservative Party by the riding of Victoria in New Brunswick.

On January 15, 1907, he was appointed a member of the Senate of Canada for the senatorial division of Victoria, New Brunswick.

Mount Costigan in Banff National Park is named in his honor.