Born in England, Johnson came to New Brunswick with his father at a very early age. Educated at the Northumberland County Grammar School, he was called to the bar in 1840. Elected to the Legislative Assembly in 1850, he entered the cabinet four years later and served successively as Solicitor General, Postmaster General, Speaker and Attorney General. A Father of Confederation, he was a delegate to both the Charlottetown and Quebec Conferences of 1864 and the London Conference of 1866. He was elected as Liberal representative of Northumberland to the House of Commons in 1867. He died at Chatham.