In 1817 he sailed, in company with a colleague, the Rev.Ebenezer Burgess, to explore the coast of Africa in search of the best site for a colony.

On the return voyage he died, and his body was committed to the sea: a "little man," to whom were granted only five years of what men call "active life"; but he had fulfilled his vow, and the ends of the earth had felt his influence for the advancement of the kingdom of the Lord Jesus Christ.
The enterprise of African colonization, already dear to Christian hearts for the hopes that it involved of the redemption of a lost continent, of the elevation of an oppressed race in America, of the emancipation of slaves and the abolition of slavery, received a new consecration as the object of the dying labors and prayers of Mills.

It was associated, in the minds of good men, not only with plans for the conversion of the heathen, and with the tide of antislavery sentiment now spreading and deepening both at the South and at the North, but also with "Clarkson societies" and other local organizations, in many different places, for the moral and physical elevation of the free colored people from the pitiable degradation in which they were commonly living in the larger towns.

Altogether the watchmen on the walls of Zion saw no fairer sign of dawn, in that second decade of the nineteenth century, than the hopeful lifting of the cloud from Africa, the brightening prospects of the free negroes of the United States, and the growing hope of the abolition of American slavery.[258:1] Other societies, national in their scope and constituency, the origin of which belongs in this organizing period, are the American Education Society (1815), the American Sunday-school Union (1824), the American Tract Society (1825), the Seamen's Friend Society (1826), and the American Home Missionary Society (1826), in which last the Congregationalists of New England cooeperated with the Presbyterians on the basis of a Plan of Union entered into between the General Assembly and the General Association of Connecticut, the tendency of which was to reinforce the Presbyterian Church with the numbers and the vigor of the New England westward migration.

Of course the establishment of these and other societies for beneficent work outside of sectarian lines did not hinder, but rather stimulated, sectarian organizations for the like objects.