Early in his career, Brearley expressed support for the institution of slavery. In a 1774 letter, he wrote that "slavery is no evil, but a blessing." He argued that slavery was necessary for the economic prosperity of the United States and that it provided African Americans with a better life than they would have had in Africa.
However, Brearley's views on slavery began to change in the late 1770s. He was influenced by the growing movement for abolition, and he came to believe that slavery was morally wrong. In 1780, he wrote that "slavery is a monstrous institution, repugnant to the principles of humanity."
Brearley's changing views on slavery are evident in his actions as a politician. In 1787, he served as the president of the Pennsylvania State Constitutional Convention. He helped to draft a state constitution that included a provision prohibiting the importation of new slaves into Pennsylvania. In 1788, he served as a delegate to the United States Constitutional Convention. He supported the inclusion of a clause in the Constitution that prohibited the federal government from abolishing slavery in the Southern states.
Brearley's views on slavery were ultimately shaped by his complex personal experiences and by the changing social and political climate of the late 18th century. He evolved from a supporter of slavery to an opponent, and his actions helped to lay the foundation for the eventual abolition of slavery in the United States.