Cooper was described by Thomas Jefferson as "one of the ablest men in America" and by John Adams as "a learned ingenious scientific and talented madcap."

"[1] His ideas were taken very seriously in his own time: there were substantial reviews of his writings, and some late eighteenth-century critics of materialism directed their arguments against Cooper, rather than against the better-known Joseph Priestley.

[8]: 150  In October 1790 the Manchester Constitutional Society was set up, with Cooper, author of Letters on the Slave Trade (1787), and other members such as Thomas Walker, noted as radicals and abolitionists.

The whole radical group resigned en masse, in 1791, when the Literary and Philosophical Society refused to send Priestley a message of sympathy.

In the rapid developments stemming from the French Revolution, Cooper was sent to Paris in 1792 with James Watt Jr., by the Constitutional Society of Manchester.

They travelled with an introduction from Walker to political circles through Jérôme Pétion de Villeneuve, and another to a man of science, Antoine Lavoisier, from Priestley.

Cooper was for some purposes a representative of the British democratic clubs to those of France, but the situation on both sides of the Channel was by now becoming complex.

Like his friend Priestley, who was then also living in Northumberland, he sympathized with the Jeffersonian Republicans, and took part in the agitation against the Alien and Sedition Acts.

[12] On 26 October 1799, the Reading, Pennsylvania Advertiser published a strong attack he wrote against President John Adams.

"[13][14] In 1806, Cooper was appointed a land commissioner and succeeded in overcoming the difficulties with the Connecticut claimants in Luzerne county.

It is a country most tolerant in theory, and most bigoted in practice", not that this made him feel obliged to return to Mother England.

Before his college classes, in public lectures, and in numerous pamphlets, he constantly preached the doctrine of free trade, and tried to show that the protective system was especially burdensome to the South.

Not surprisingly, the evangelical Charles Colcock Jones, who was a missionary to slaves as well as a professor at Columbia Theological Seminary, was unimpressed with Cooper.

"[17] He exercised considerable influence in preparing the people of South Carolina for nullification and secession; in fact he preceded Calhoun in advocating a practical application of the state sovereignty principle.

He wrote that "negroes are men; susceptible of the same cultivation with ourselves", claimed that "as Englishmen, the blood of the murdered African is upon us, and upon our children, and in some day of retribution he will feel it, who will not assist to wash off the stain".

[5][2] After moving to South Carolina in 1819, he disavowed his previous anti-slavery views and purchased several enslaved families[5] and began to defend slavery on Benthamist utilitarian grounds.

[4][2] In an 1826 essay, Cooper wrote that the "emancipation of the Slaves, would surely convert them into idle and useless vagabonds, and thieves; as every Southern man conversant with negro habits and propensities well knows".

As a philosopher he was a follower of David Hartley, Erasmus Darwin, Priestley, and François-Joseph-Victor Broussais; he was a physiological materialist, and a severe critic of Scottish metaphysics.

[12] The last years of his life were spent in preparing an edition of the Statutes at Large of the state, which was completed by David James McCord (1797–1855) and published in ten volumes (1836–1841).