Consists of journals, illustrations, and documents of Captain John Matthews (d. 1798), a British naval officer involved in the transatlantic commerce in enslaved Africans in Sierra Leone in the 1780s and 1790s. Four detailed journals document Matthews's employment as an agent for the African Company of Merchants between 1785 and 1787; as captain of the HMS Vulcan and the HMS Courageux in the Mediterranean Sea during the 1793 campaigns of the French Revolutionary Wars; and as captain of the HMS Maidstone, a British patrol ship monitoring trade in Sierra Leone and the Caribbean in 1797 and 1798. Also present are watercolor illustrations of colonial forts along the West African coast and several personal and family documents of John Matthews, including an anti-abolitionist deposition Matthews gave regarding the slave trade.

The majority of the collection, including three of the journals, pertains to Matthews's involvement in the slave trade in Sierra Leone and the Windward Coast region of West Africa, first as an agent of London-based slave merchants, Samuel Hartley and Company and the African Company of Merchants, and later, as an agent of the British government policing transatlantic commerce. The region where Matthews was active is also sometimes referred to as the Grain Coast or Rice Coast and includes present-day Liberia and Côte d'Ivoire, as well as Sierra Leone. Enslaved Africans from these regions were one of the largest captive groups imported into South Carolina and Georgia during the 18th century. Their descendents include some of the Gullah people of the Lowcountry region of those states.

The two earliest journals cover the same time period (1785-1787) and activities Matthews describes in his book, A Voyage to the River Sierra-Leone, on the Coast of Africa; Containing an Account of the Trade and Productions of the Country, and of the Civil and Religious Customs and Manners of the People... (London, 1788). These journals, which provide additional information not published in Matthews's book, document Matthews's travels inland and along the coast of Sierra Leone, his negotiations with African kings and fellow slave traders, his work supplying ships and stocking them with captive Africans, and the resistance of those captured to their enslavement and transport. Also included are retained copies of several letters and agreements related to the trade. Another journal from 1797 and 1798 records Matthews's official duties as captain of the HMS Maidstone in both West Africa and the Caribbean, as well as his struggles with his crew's discipline, drunkenness, and poor health on board the ship; his visits to leading figures in coastal settlements, and his observations and theories about African communities, politics, law, and cultural practices.

Matthews's journals emphasize the Mande-speaking peoples of West Africa, whom he refers to as "Mandingoes," the practice of domestic slavery within Africa, religious wars between Muslim and non-Muslim kingdoms, and his musings on the "disposition" and "nature" of African peoples. Though Matthews makes claims about African communities living in the interior of the continent, he also notes that he only spoke the language of the coastal Africans. His observations are therefore based on his own interpretations of what he witnessed, as well as the perspectives of coastal Africans about other African communities living further inland. In addition to the frequent sketches of coastlines within the journals, there are eight watercolor paintings depicting colonial forts and vessels along the coast of West Africa and the Sierra Leone River, four of which are early sketches for engravings that appeared in the second edition of Matthews's book. Also of note is a manuscript transcription of a testimony Matthews gave on Sierra Leone during British parliamentary debates over the abolishment of the slave trade in the late 1780s.

To a lesser extent, the collection also documents Matthews's participation in the Mediterranean fleet of British Admiral Samuel Hood (1724–1816) during the French Revolutionary Wars. One journal, with entries spanning from May through September 1793, records his activities as the captain of the HMS Vulcan and the HMS Courageux leading up to and during the early stages of the Siege of Toulon (1793). The journal contains orders he received, including his reassignment, as well as his observations and sketches of the various Mediterranean coastlines he passed on his way to Toulon.

Other documents include Matthews's admission as a freeman of Liverpool, his appointment as captain of the HMS Maidstone, and a medical license belonging to a descendent.