When American Presbyterian and Congregationalist missionaries arrived in the Gabon Estuary in the 1840s, they entered a world marked by vibrant commerce; violence and inequality; widespread slavery and slave-trading; British, French, and U.S. Anti-Slavery Patrols; and incipient French colonialism. This article draws on the published accounts by two U.S. missionaries, John Leighton Wilson, who served in Gabon from 1842 to 1851, and Robert Hamill Nassau, who worked on Corisco Island, the Gabon Estuary and Ogowe River, and the southern Cameroon coast from 1861 to 1906. Together, their writings provide insights into early colonialism and especially the long decline of enslavement and slave trading. While Wilson witnessed the establishment of Libreville in the 1840s, Nassau encountered slave trading first on Corisco and later on the Ogowe during the period of French colonial exploration. Both men, shaped by their African experiences as well as their respective social locations in the United States, held strong views on African domestic slavery and the slave trade. Wilson, from the South, was an ambivalent abolitionist who railed against the Atlantic Slave trade while hesitating to denounce slavery and racial inequality in his native South Carolina. Nassau, from New Jersey and educated at conservative Princeton University, was prompted above all by the missionary impulse. He sought to convert and “uplift” formerly enslaved Africans while nevertheless underlining their “servile” characters and benefitting from their labor as docile, socially vulnerable mission workers.