John Singleton Copley, America's most important colonial painter, was born in Boston of Irish parents. In 1748 Copley's widowed mother married Peter Pelham, a painter and engraver. Copley's stepfather probably gave him some art lessons but died when Copley was only 13. In later years the painter claimed he was self–taught.
Copley, who was extremely observant, presumably learned about art largely by watching other English–trained painters who were working in the New World and by studying engravings imported from Europe. Much more important was his innate ability to record details objectively and to suggest character. Gilbert Stuart would later say of the uncompromising realism in Copley's Epes Sargent, "Prick that hand and blood will spurt forth."
About 70 years old when he posed for Copley, Sargent had dropped out of Harvard College to enter business in his native Gloucester. After the death of his first wife, this prosperous merchant and shipowner married a rich widow from Salem. Copley's portrayal shows him nonchalantly leaning on a marble pedestal as a symbol of prestige; since carved stone monuments were rather rare in the colonies, this imaginary device must be borrowed from European prints of potentates.
Such penetrating likenesses made Copley the best–paid artist in colonial America. By shipping some of his canvases to London for criticism, Copley soon became known in England.
More information on this painting can be found in the Gallery publication American Paintings of the Eighteenth Century, pages 24-28, which is available as a free PDF at https://www.nga.gov/content/dam/ngaweb/research/publications/pdfs/american-paintings-18th-century.pdf