In 1768, he returned to the Irish House of Commons as a Member of Parliament for Armagh Borough, in order to discharge the duties of Chief Secretary for Ireland.

After losing control of the fortifications on Hospital Hill — an essential defence position located on a prominence overlooking the island capital St. George's—Macartney chose to surrender unconditionally and was taken prisoner to France.

Popham also advocated measures for the naming and lighting of streets, for the regular registration of births and deaths and for the licensing of liquor, arrack and toddy shops.

After being created Earl Macartney in the Irish peerage in 1792, he was appointed the first envoy of Britain to Qing China, after the failure of a number of previous embassies, including Cathcart's.

He led the Macartney Embassy to Beijing in 1792, with a large British delegation on board a 64-gun man-of-war, HMS Lion under the command of Captain Sir Erasmus Gower.

The embassy was ultimately not successful in its primary aim to open trade with China, although numerous secondary purposes were attained, including a first-hand assessment of the strength of the Chinese empire.

[12][incomplete short citation] The Macartney Embassy is historically significant because it marked a missed opportunity by the Chinese to move toward some kind of accommodation with the West.

Gower also left a more personal record through his private letters to Admiral John Elliot and Captain Sir Henry Martin, 1st Baronet (Comptroller of the Navy).

[15] On 23 December, Macartney recorded in his journal: "I have given up my projected visit to Japan, which (though now less alluring in prospect) has always been with me a favourite adventure as a possible opening of a new mine for the exercise of our industry and the purchase of our manufactures".

[16] On his return from a confidential mission to Italy in 1795, he was raised to the British peerage as Baron Macartney, of Parkhurst in the County of Surrey and Auchinleck in the Stewartry of Kirkcudbright,[17] and at the end of 1796 was appointed governor of the newly acquired territory of the Cape Colony,[18] where he remained until ill health compelled him to resign in November 1798.