In 1779 Newton was invited by John Thornton to become Rector of St Mary Woolnoth, Lombard Street, London, where he officiated until his death, but where he also contributed to the work of the Committee for the Abolition of the Slave Trade, formed in 1787. Newton was one of only two evangelical Anglican priests in the capital, and he soon found himself gaining in popularity amongst the growing evangelical party. He was a strong supporter of evangelicalism in the Church of England. He remained a friend of Dissenters (such as Methodists and Baptists) as well as Anglicans.
Young churchmen and people struggling with faith sought his advice, including such well-known social figures as the writer and philanthropist Hannah More, and the young William Wilberforce, a Member of Parliament who had recently suffered a crisis of conscience and religious conversion while contemplating leaving politics. The younger man consulted with Newton, who encouraged Wilberforce to stay in Parliament and “serve God where he was.”
In 1788, 34 years after he had retired from the slave trade, Newton broke a long silence on the subject with the publication of a forceful pamphlet Thoughts Upon the Slave Trade, in which he described the horrific conditions of the slave ships during the Middle Passage. He apologised for “a confession, which … comes too late … It will always be a subject of humiliating reflection to me, that I was once an active instrument in a business at which my heart now shudders.” He had copies sent to every MP, and the pamphlet sold so well that it swiftly required reprinting.
Newton became an ally of William Wilberforce, leader of the Parliamentary campaign to abolish the African slave trade. He lived to see the British passage of the Slave Trade Act 1807, which enacted this event.