He first stayed in Basel, then in Amsterdam, before settling in London with his uncle Isaac Guiguer who was an associate of Nicolas Tourton, where he learned languages and banking.

In no small party due to the fact that Thellusson was a Genevan Huguenot, the two Catholic Guiguer nephews successfully attacked the will.

The two partners, of very different generations and characters, did not get along well and, in October 1740, the company was dissolved their business feud was finally eventually arbitrated by the Small Council of Geneva in 1748.

[5] The English branch of his male descendants, the only one surviving in the 21st century, has held the title of Baron Rendlesham in the Peerage of Ireland since 1806.

[9] She was a daughter of Abraham le Boullenger, Lord of Rixdorp, a Huguenot merchant who had emigrated from Rouen in 1685, and Anne van der Hulst, who was Dutch.

[1] Through his son Peter, he was a grandfather of Anne Thellusson (1774–1849), who married Vice-Admiral William Lukin Windham in 1801,[11] and had descendants including Diana, Princess of Wales, as well as her sons William, Prince of Wales heir to the British throne, and Prince Harry, Duke of Sussex.