Diplomat and soldier. The third son of General William Cathcart (1755 - 1843), the 1st Earl of Cathcart, younger brother of General Charles Cathcart (1783 - 1859), and elder brother of Lieutenant-General Sir George Cathcart (1794 - 1854), like them Cathcart joined the army and rose to the rank of Colonel.
He was appointed Minister Plenipotentiary at the Court of St Petersburg and Secretary of the British Embassy in Russia (1820) and was created a Knight of the Russian Order of St Anne. He went on to serve as Envoy Extraordinary to the German Confederation in Frankfurt (1824-26).
He married Jean McAdam of Craigengillan, whose father had committed suicide in 1805 and left his estate in trust. Cathcart changed his name to McAdam-Cathcart to ensure he could gain title to the estate, which lies near Dalmellington.
The couple died childless.