John Mercer Johnson was born in October, 1818, in Liverpool, England, the son of John M. Johnson. The family emigrated to Chatham, Northumberland County, New Brunswick where Mr. Johnson, Sr. was engaged in the lumber business and became sheriff of Northumberland County.

John Johnson, Jr. was educated at the Chatham Grammar School. He studied law and was admitted to the Bar of New Brunswick as an attorney October 13, 1838 and as a barrister in October 1840. He located at Chatham where he practiced law and became one of the leading lawyers on the North Shore. On October 9, 1843 he married Miss Henrietta Shireff, daughter of A. D. Shireff of Miramichi. By this marriage there were several children.

He was first elected to the House of Assembly of New Brunswick as one of the members for Northumberland County at the general election of July 2, 1850. He was re-elected at the general elections of June 20, 1854, June 27, 1856, May 5, 1857 and June 1861. He was also elected at by-elections held November 14, 1854, July 22, 1857 and October 28, 1862 made necessary by his acceptance of portfolios. He sat continuously as a member until the dissolution of the Legislature in 1865.

At the general election of March 18, 1865, he was defeated on the issue of Confederation, of which he was in favour. He was again elected to the House at the general election of May 25, 1866 on the same issue and sat as a member until June 1867 when he resigned his seat in order to enter federal politics.

On October 31, 1854, he was sworn in as a member of the Executive Council of New Brunswick and received the appointment of Solicitor General in the administration of Hon. Charles Fisher. He held this appointment until Fisher's administration resigned in 1856. On June 1, 1857 he was again sworn in as a member of the Council and received the portfolio of Postmaster General. He resigned this portfolio in November 1858 and was appointed Minister Without Portfolio, resigning the latter appointment in 1859.

In October 1862 he again joined the Executive Council when he accepted the portfolio of Attorney General in the Tilley administration.

He was first elected Speaker of the House of Assembly on February 11, 1859 after Hon. James A. Harding resigned to accept the position of High Sheriff of the City and County of Saint John. He was re-elected Speaker of the House of Assembly on February 12, 1862 and retained the position through that session until his appointment as Attorney General later in the year.

He was a New Brunswick delegate to the Charlottetown Conference in 1864 at which Confederation was proposed; he was a delegate to the Quebec Conference in the fall of 1864 and to the London Conference in 1866 when The British North America Act was drawn up and passed by the Imperial Parliament. Because of his participation in these conferences and the active role he played in bringing about the union of the provinces, he is referred to as one of the Fathers of Confederation.

At the first federal general election in 1867, he was elected to the new House of Commons for the County of Northumberland and sat as a member until his death.

He died November 8, 1868 at his residence in Chatham at the age of 50.

Source - Speakers of the Legislative Assembly, Province of New Brunswick, 1786-1985. 1985, Legislative Assembly, Province of New Brunswick, Office of the Clerk, Fredericton, N.B.