John Mercer Johnson was born in 1818 in Liverpool, England. He was brought to New Brunswick by his father, where he became a lawyer in 1850. He was later Solicitor General, Minister for the Stations, and Attorney General. He was delegated to the conference of Quebec in 1864 and that of London in 1866. After the confederation in 1867, Johnson was elected to the new House of Commons of the federal Parliament.
Johnson was educated at a grammar school in Northumberland before he studied law. In 1840, he was called to the bar of New Brunswick. At the conference of Charlottetown and the two later conferences discussing the confederation, Johnson vigorously asserted the fact that the provincial governments should not hold any power above the course of the county or zone.