In 1774, he went into business as a lawyer, and in 1781 married Jane, second daughter of William Griffies, a Liverpool tradesman; they had seven sons and three daughters. Roscoe had the courage to denounce the trans-Atlantic slave trade in his native town, where, at that time, a significant amount of the wealth came from slavery. Roscoe was a prominent Unitarian. His outspokenness against the slave trade meant that abolitionism and Unitarianism were linked together in the public mind. He was a member of Renshaw Street Unitarian Chapel.