From the early 1760s Thellusson worked with the Fonblanque family, from 1763-6 in partnership with Anthony and the widow of his brother John, with each partner contributing £5,000 to the business.
When Anthony died in late 1766, Thellusson operated independently for a while before establishing a new and long-lasting partnership with John Cossart, the former chief clerk at Fonblanque and Thellusson, in which he was the dominant partner with a large majority share of 15/16ths of the business.
Source:Polden, P 2002 Peter.Thellusson´s.Will.of.1797.and.its.Consequences.on.Chancery.Law. Lampeter: Edwin Mellen Press, 41, 42, 44, 47-48, 63; Thorne, R G (ed.) 1986 The.History.of.Parliament:.The.House.of.Commons,.1790-1820,.Vol.5.Members.Q.Y.
London: Secker & Warburg, 362

Premises in Tower Street, Lime Street and


Slavery
In 1765 in partnership with John & Anthony Fonblanque, Huguenot merchants of Great Tower Street, London, Thellusson invested in the slaving venture of the ship Lottery, William Hayman, Master, a ship bound for Grenada via the Windward Coast carrying 172 slaves; Source: The Trans-Atlantic Slave Trade: A Database on CD-ROM, by David Eltis, Stephen D. Behrendt, David Richardson, and Herbert S. Klein, published by Cambridge University Press, (1999).