Peter Mitchell was born on January 4, 1824, in Newcastle, Northumberland, New Brunswick.

He was a lawyer, politician, shipbuilder, author, office holder, and one of the Fathers of Confederation. He attended the Newcastle Grammar School. He studied law and was called to the bar of New Brunswick in 1848, practising until 1853 when he withdrew from the legal profession in favour of pursuing a career in the lumbering and shipbuilding industries, at first in a partnership with John Haws, and later on his own. He participated in the activities of the Northumberland Agricultural and Highland societies. In 1855, he was one of the directors of the Newcastle Grammar School. He was also appointed both a Justice of the Peace and a Justice of the Inferior Court of Common Pleas. He was named a captain in the 1st Battalion of militia in 1865. Mitchell was an exceptionally able politician, and throughout the early 1860s, he was one of the most vocal advocates of Confederation. In 1864, he took part in the Quebec Conference. He was later a delegate to the London Conference, which drafted the British North America Act, and is thus one of the Fathers of Confederation. After Confederation, he was called to the Senate and appointed as Canada's first Minister of Marine and Fisheries in the administration of Sir John A. Macdonald. Mitchell resigned from the Senate in 1872 to run for a seat in the House of Commons of Canada. He abandoned the Liberal-Conservative Party of Macdonald and declared himself an independent Member of Parliament. He sought and won the Northumberland County seat in the House of Commons where he sat for a total of fifteen years (1872-78 and 1882-91). In 1885, he purchased the Montreal Herald and Daily Commercial Gazette and used them to attack the policies of both Liberals and Conservatives.

In 1853, he married Isabella Gough Carvell (1822–1889). He died on October 25, 1899, in Montreal, Quebec.