Lumley & Co. purchased her in prize and employed her for two voyages as a slave-ship in the triangular trade in enslaved people.

Frederick enters Lloyd's Register in the Supplemental pages in 1805 with M'Donald, master, Thomas Lumley, owner, and trade London-Africa.

1st voyage transporting enslaved people (1805–1806): On 16 September 1805, MacDonald sailed to the Gold Coast to acquire captives.

On his return to London, M'Donald reported that some 25 Guineamen had been captured on the Coast, together with HMS Favourite.

2nd voyage transporting enslaved people (1807–1808): The next year he sailed to the Bight of Benin, leaving London on 1 January 1807.

She arrived at Port Jackson on 21 October, with merchandise,[8] having called in at Rio de Janeiro on the way.

[9] Frederick, E. Bunker, master, returned to Port Jackson on 20 August 1811, from the fishery, with 55 tons of sperm oil for the London market.

[11] On 27 September 1812, Frederick left St Helena with the whalers Admiral Berkley and Argo, under escort by HMS Galatea, returning from the Indian Ocean.

It further reported that Adndromache and Briton had chased Sans Souci for 12 hours before catching her.

[18] Rolla shared the salvage money for Frederick with HMS Dublin and Inconstant.

The last detailed report of a whaling voyage has Frederick leaving Britain on 30 September 1816 and returning on 11 November 1817.