John Mercer Johnson (October 1, 1818 – November 8, 1868) was a politician in the Province of New Brunswick, Canada and a Father of Confederation. He represented Northumberland in the Canadian House of Commons from 1867 to 1868 as a Liberal member.
He was born in Liverpool, England, and brought at an early age to Chatham, New Brunswick, by his father, where he became a lawyer in 1850. He represented Northumberland County in the Legislative Assembly of New Brunswick from 1850 to 1865 and from 1866 to 1867. Johnson was Solicitor General from 1854 to 1856, postmaster general from 1857 to 1859, speaker for the house from 1859 to 1862 and Attorney General from 1862 to 1865. He was delegated to the Quebec Conference in 1864 and that of London in 1866. After Confederation in 1867, Johnson was elected to the new Canadian 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 and became a fully licensed lawyer. In 1850 he became a liberal member for his county and later in 1854 when his party came to power under Fisher his name appeared among those in the "Smashers" administration. At the Charlottetown Conference and the two later conferences discussing the confederation, Johnson vigorously asserted that the provincial governments should not hold any power above the course of the county or zone.
He died in office at Chatham, New Brunswick at the age of 50.