Brunias was born in Rome c. 1730; the exact day and month of his birth is uncertain. His first name has been recorded in various ways, including Abraham, Alexander, August, or Austin, while his surname has been recorded as Brunais and Brunyas. He became a student at the Accademia di San Luca, one of the most prestigious art institutions in Rome. In 1752, he exhibited an oil painting, and in 1754 he won Third Prize in the Second Class for painting. Brunias met the prominent Scottish architect Robert Adam, who was on a Grand Tour studying the "magnificent ruins of Italy" between 1754 and 1756. Adams employed Brunias in his workshop in Rome, and Brunias accompanied Adams on his return to Britain in 1758. Brunias worked as a draughtsman and painter on many of Adam's building projects in Britain. Adam, praising his works, called Brunias a "bred painter". His murals and paintings covered the interior walls of several stately homes of the British upper class. Surviving examples of Brunias' early work include five paintings in the classical style, which were commissioned to decorate the breakfast room at Kedleston Hall, now housed at the Victoria and Albert Museum. By 1762, Brunias was residing in Broad Street, Carnaby Market, London and in 1763 and 1764, he exhibited at the Free Society of Artists.
