He also joined Ramsay's studio in the 1760s as its principal draughtsman, helping to produce many of the coronation portraits of George III and Queen Charlotte.

His Dictionary of National Biography entry states that "He portrayed his sitters with integrity in an honest natural style, thereby consolidating a recognizably Scottish tradition of portraiture".

[1] As well as producing his own paintings, Martin copied them himself in highly-praised mezzotints, such as those of Lady Frances Manners (1772), impressions of which may be seen in the British Museum, under catalogue entries 1887 0406 87 and 1887 0406 142.

[2] In 1780, Martin returned to Edinburgh (although not selling his home in High Street, Dartford, until 1782), a move signalled by his admittance to the Royal Company of Archers and a rare full-length portrait he painted of its president, Sir James Pringle of Stichill (1791–4).

Martin died in 1797 at his home, 4 St James Square, Edinburgh, and was buried on 3 January 1798, in the graveyard at South Leith Parish Church.