After some years of employment by Robert Adam, who had first taken on Brunias as an architectural draughtsman in Rome in 1756, Brunias accompanied Sir William Young (on "the first of six West Indian Voyages made by Sir William in his public employ") as draughtsman to Barbados in 1764. Young had been appointed President of the Commission for the Sale of Lands in the Ceded Islands of Dominica, St Vincent, Grenada, and Tobago (the recently captured Southern Caribbees) in 1764, and was appointed first British Governor of Dominica in 1770. Brunias's work from this time on concentrates on subjects in the West Indies, in particular in Dominica, St. Vincent (where Young had purchased land), Saint Christopher and Barbados, painted for his patron and for the numerous white oligarchs who ran estates on the islands, such as Sir Patrick Blake and Sir Ralph Payne (Captain-General and Governor-in-Chief of the Leeward Islands). Brunias appears to have returned to England in 1773 and was resident in Soho when he exhibited Dominican subjects at the Royal Academy in 1777 and 1779. First editions of engravings after his West Indian pictures were 'Published by the Proprietor, No. 7 Broad Street, Soho' (the address from which he submitted his two R.A. exhibits in 1779) in 1779-80. He returned to work in the West Indies in the early 1780s (Dominica and St Vincent were returned to Britain by the Treaty of Versailles in 1783) and settled there until his death in Roseau, Dominica, in April 1796.
Sir William Young's office as Receiver and Governor closed on October 1774 and he returned to England. On his death in 1788 his son William inherited the baronetcy and estates in Tobago (where he was appointed Governor), Betsy's Hope in Antigua, and Calliaqua and Pembroke in St Vincent.