Bonnar showed an early aptitude for drawing, and was apprenticed to one of the leading decorators of the time.
When George IV visited Edinburgh in 1822, Bonnar assisted David Roberts in decorating the Assembly Rooms for the grand state ball which was given in honour of the occasion.
Shortly afterwards some signboards painted by him attracted the notice of Captain Basil Hall, who sought out and encouraged the young artist.
In the year 1824 his picture of 'The Tinkers ' established him as a favourite with the public, and shortly after the formation of the Royal Scottish Academy in 1830 he was elected one of its members.
His monument is almost identical to that of his father in Dean Cemetery who had died only a few years earlier, and is therefore presumed to be by David Watson Stevenson.