The sitter was the eldest son of Sir Peniston Lamb, 1st Viscount Melbourne (1748-1828), and Elizabeth (1749-1818), daughter of Sir Ralph Milbanke, 5th Bt., of Halnaby Hall, Yorkshire (see lot 44). From an early age Peniston was Lord Melbourne's idol and was said to resemble him 'in many points, both moral and physical' (see M. Boyle, Biographical Catalogue of the Portraits at Panshanger the seat of Earl Cowper, K.G., London, 1885, p. 320). Peniston showed no predilection for politics or public life, which his brother William delighted in. He was seen as:
'gentle-hearted and engaging; every tenant on the estate, every servant in the house, every dog and horse in the stable, loved him. He was a capital shot, and rode well to hounds, while quite a little fellow; and Lord Melbourne was never tired of telling how 'Pen' had led the field' (see M. Boyle op. cit.).
His affair with Mrs Musters, half a generation his senior, the erstwhile subject of portraits by Reynolds and Romney as well as Stubbs, caused much contemporary comment. He was Member of Parliament for Hertfordshire and died unmarried in 1805. Sir Joshua Reynolds painted the sitter twice: first, in a full-length, exhibited at the Royal Academy in 1773, of him with his mother Elizabeth, Viscountess Melbourne, and second, in a portrait with his two younger brothers William, later 2nd Viscount Melbourne, and Frederick James, later 3rd Viscount Melbourne. He was also painted by John Hoppner and Maria Cosway.