His grandfather, Issac de Thellusson, became Genevan ambassador at Paris to the Court of Louis XV,[3] where his uncle, George, founded a banking house.
He was the last of his three brothers to remain active in the family business, taking as his partners his nephew George Thellusson and one William Mitchell of Serjeant's Inn.
[2] "Thellusson was noticed in the House before he obtained a seat there, as chairman of a meeting of claimants on property confiscated in Martinique whose memorial to the Duke of Portland caused controversy in the debates of May and June 1795.
The following day his brother Peter asked Windham to inform Pitt that "if the family were to incur fresh trouble and expense at the Southwark by-election they required a 'reasonable assurance' of a peerage for their father or himself: otherwise George would leave Tierney to walk over and 'take a quiet seat for a borough in the Isle of Wight'.
He, instead, exploited the disarray in the affairs of the dying Richard Barwell to "secure his unopposed return for Tregony :in place of one of the sitting Members, Lord Blandford, who was seeking re-election after appointment to office.