Justin McCarthy, the second child and elder son of Michael Francis McCarthy, clerk to the Cork city magistrates, and Ellen FitzGerald, was born at Dunmanway, near Cork, on 22nd November 1830. After leaving school he found work on The Cork Examiner.
In 1852 he moved to England and served as secretary to the parliamentary commission on Fairs and Markets. He also worked for the Northern Daily Times in Liverpool and after moving to London in 1860, he was on the staff of the radical The Morning Star. Four years later he was appointed editor of the newspaper.
McCarthy's first novel, Paul Massie: A Romance, was published in 1866. This was followed by The Waterdale Neighbours (1867). The following year he resigned as editor of The Morning Star in order to travel to the United States. He returned in 1871, where he subsequently wrote for several periodicals including The Fortnightly Review, The Contemporary Review and The Nineteenth Century. Soon after his return he also became a leader writer on The Daily News, where he was employed for the next twenty-three years.
McCarthy also continued to write novels and His Dear Lady Disdain (1875) was especially popular. However, his greatest success was with his non-fictional A History of our Own Times (1878). He was very interested in Irish politics and in 1877 he joined the Westminster Home Rule Association. On 4th April 1879 he was returned unopposed as a home-ruler at the by-election for County Longford. In the House of Commons he was a strong supporter of Charles Stewart Parnell.
According to his biographer, S. L. Gwynn: "Although regularly in attendance in the House of Commons, McCarthy played a minor part in the political disputes of the 1880s and was not identified with any significant cause, although he held fast to his liberal opinions on most questions, becoming quite well known to Gladstone. At the general election of 1885 he contested both North Longford and Londonderry City, narrowly losing the latter. At the general election of 1886, however, he captured Londonderry City."
McCarthy played an important role in the Parnell divorce case. William Gladstone threatened to resign if Parnell were to be re-elected. McCarthy, who had worked closely with Gladstone and the Liberal Party in the past, led the withdrawal of the majority of nationalist MPs, who thereafter, under his chairmanship, were generally known as "anti-Parnellites". He resigned from the chairmanship of the Irish National Federation in February 1896.