Sir Robert Mackreth (?1725–1819), of Ewhurst, Hampshire, was an English club owner, money lender, speculator and politician.
He acquired a vintner's business in St. James's Street, and became an assistant of Robert Arthur, the original proprietor of White's, who on his death, 6 June 1761, left the property to Mackreth, then about to marry his only child, Mary Arthur (the wedding took place in October).
[2] From this point Mackreth concentrated on financial speculations, with sidelines as a loan shark and bookmaker: Gilly Williams, writing to George Selwyn in 1768, mentions him as dealing heavily in the bets for and against the success of John Wilkes in the election for the City of London that year.
In October 1774 he was nominated for the pocket borough of Castle Rising by the Earl of Orford, who had found him useful in business, and was in his debt.
Eldon ignored the challenge, but brought an action for assault against Mackreth, who was sentenced by the court of King's Bench to six weeks' imprisonment and a fine, for a breach of the peace.