Knapton was born in London, one of four sons of James Knapton. He was apprenticed to Jonathan Richardson from 1715 to 1722, and in 1720 was a founding subscriber to the academy of St. Martin's Lane established by Louis Chéron and John Vanderbank. He spent three years in practice on his own, and was one of the six young founders of the Roman Club in 1723. Thereafter he spent seven years in Italy, from 1725 to 1732, where he acquired considerable knowledge of the old masters. He was a founding member of the Society of Dilettanti, formed in Rome in the early 1730s, and as its official portrait painter, executed between 1741 and 1749 twenty-three portraits of members of the society in a variety of fancy dress; these are his principal claim to fame.
Although Knapton painted such large canvases as the group portrait of Augusta, Princess of Wales, and her children, he was best known for his work in pastel, of which he was the finest practictioner in Britain in the 1730s and 1740s. He executed some of the portraits of historical worthies that were engraved for Thomas Birch's Illustrious Persons of Great Britain, published by his brothers in two volumes in 1743 and 1751; and, as a distinguished connoisseur, he was asked to catalogue the pictures at Althorp (1746) and survey the royal collection (1750). Knapton seems to have given up painting after about 1755. He succeeded Stephen Slaughter as surveyor of the king's pictures in 1765, and died in Kensington in December 1778.
[Hayes, John. British Paintings of the Sixteenth through Nineteenth Centuries. The Collections of the National Gallery of Art Systematic Catalogue. Washington, D.C., 1992: 147-148.]