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.
On inquiry into Tierney’s petition Thellusson was found guilty of treating offences and his election was declared void, 11 Nov. 1796. The following day Peter Thellusson 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’.
Thellusson, who bought Aldenham in 1799 and sold it to the trustees of his father’s infamous will in 1805,
He did not find a seat in the 1806 Parliament, but in 1807 stood, with the good wishes of the Portland ministry, for the venal borough of Barnstaple and topped the poll. He voted for Folkestone’s motion on Wellesley’s conduct in Oudh, 15 Mar. 1808, and against Perceval’s exculpatory resolution on the Duke of York scandal, 17 Mar. 1809. A month later he failed to secure re-election as an East India Company director, but in January 1810 Richard Ryder reported that he had ‘avowedly changed sides’
George Thellusson was the last of the three brothers to remain active in the family business, taking as his partners his nephew George Thellusson and one William Mitchell of Serjeants’ Inn. The firm, now known as Thellusson, Nephew & Co., had moved from 34 Little Eastcheap to 24 Old Jewry by 1811 and the following year was located at Meeting House Court. In his will, dated 21 Jan. 1811, he released Mitchell from a personal debt and ‘most earnestly’ solicited him ‘to recover as speedily as possible the considerable debts owing to my several partnerships’ and ‘give every assistance in his power to the executors of my late brother Lord Rendlesham for the like purpose’.
