Thane Alexander Campbell was born in Summerside, Prince Edward Island, on 7 July 1895, the son of Alexander and Clara (Muttart)Campbell. He married Cecilia Bradshaw (1906-1968), daughter of Melville Bradshaw on 28 February 1930. They had four children: Virginia; Alex; Harriet; and J. Melville. After the death of his first wife, Campbell was married a second time to Paula Agnes Champ.

Campbell received his early education at Summerside Public School and then attended Prince of Wales College. He went on to receive a Bachelor of Arts and Master of Arts from Dalhousie as well as a Masters of Arts from Oxford University in England where he attended Corpus Christie as a Rhodes Scholar. In1922, Campbell returned to Prince Edward Island to read law with A. C. Saunders in Summerside.

As a member of the Liberal party, Campbell first ran for political office in a 1930 by-election in 2nd Prince but was defeated. In that year, he was appointed Attorney General of Prince Edward Island, a position he held until the following year when he was elected to the legislature as the member for 1st Prince. He was reelected in 1935 and was appointed Attorney and Advocate General in Premier Walter Lea