Robert Mayne was a banker in Jermyn St.: his firm first appears in the trade directories in 1770 as Mayne and Needham; in 1774 it becomes Robert Mayne and Co.; and in 1778 Mayne and Graham. He was also an important Government contractor: his first contract, 9 Feb. 1776, held jointly with Anthony Bacon, was to victual 6,000 men in America;
He sat at Gatton on his brother’s interest, and voted steadily with North’s Administration. There is no record that he ever spoke in the House. In 1780 he contested Stockbridge, a very corrupt borough, and Colchester, where he received only 12 votes on a poll of over 600.
In 1782 his firm went bankrupt, and on 5 Aug. he committed suicide. On 7 Aug. Sir John Eliot, a Scottish physician practising in London, wrote to John Douglas, later bishop of Salisbury:
Mayne’s bankruptcy is attended with circumstances of uncommon villainy. Our friend Mac [probably James Macpherson] has not only lost above a thousand pounds, but a valuable diamond ring, which he had deposited there for safety. The box which contained it is found empty. My usual good fortune served me. I lost only fifty pounds. During the whole of the last year I had a large sum of money there and indeed till the day after the taking of St. Christopher. Then the stocks fell lower than they have ever been and I wasted the whole sum in them.
Mayne advised me so strongly to let the money remain that I suspected him, and never paid in any more to his shop.