He was a Member of Parliament (MP) from 1879 to 1900, taking his seat in the House of Commons of the United Kingdom of Great Britain and Ireland.

[5] McCarthy was first elected to Parliament at a by-election on 4 April 1879, when he was returned unopposed as a Home Rule League MP for County Longford.

[8] At the 1886 general election, he was returned unopposed in North Longford,[9] but had also stood in Londonderry City, where he was declared to have lost to the Unionist candidate by the narrow margin of 1778 votes to 1781.

England, he argued, had avoided continental revolution because in a Parliament otherwise incapable of anticipating "the wants and wishes of the country" her statesmen were shrewd enough to defer to "pressure from without".

To "the manner in which the Government resisted Catholic Emancipation, and the grudging way of at last conceding it", he ascribed much of Ireland's subsequent "discontent and disaffection".

McCarthy traced to the days of Robert Walpole and William Pulteney the origins of the contemporary English political parties which, appealing to prejudices and passions, seek to "manufacture" a public opinion of their own.

[17] He also collaborated on three novels with Rosa Campbell Praed: The Right Honourable (1886), The Rebel Rose (issued anonymously in 1888 but appeared in their joint names in two later editions under the title, The Rival Princess), and The Ladies' Gallery (1888).

[citation needed] His biographer, Liam Harte, suggests that McCarthy's award of a civil-list pension for services to literature in 1903 "confirmed his stature as an eminent Victorian, while simultaneously reinforcing many Irish nationalists' jaundiced view of him as a careerist West Briton".