He sat in the House of Commons of the United Kingdom between 1877 and 1888 as an Irish nationalist, and later Unionist, Member of Parliament.

He inherited Rockingham Estate which was a fine house built by John Nash, but altered in a less than sympathetic way in the late 19th century in order to provide more accommodation.

King-Harman stood unsuccessfully as the Home Government Association candidate in the May 1870 rerun of the December 1869 Longford by-election after the result of the first vote was overturned.

As result of Gladstone's Representation of the People Act 1884 which would extend the Irish franchise, some Orangemen were threatening violence and T. P. O'Connor complained in parliament of several politicians using inflammatory language.

They had one daughter, Frances Agnes, who married Sir Thomas Stafford, 1st Baronet, a physician and member of the Irish Privy Council.