Zachary Macaulay was born in Inveraray, Scotland, the son of a Minister of the Church of Scotland. He only had a basic education but taught himself the Classics, Latin and Greek. He worked at a merchant counting house in Glasgow. At only 16 years of age, he went to Jamaica. He worked for eight years on a sugar plantation, eventually becoming the manager. At first he was horrified by the way the enslaved were treated but became hardened to it.
On returning to Britain, he met William Wilberforce through his sister's husband, Thomas Babington. Macaulay's time in Jamaica had left a a deep impression on him and he became a member of the Society for the Abolition of the Slave Trade. In 1794, he took the post of Governor of the Sierra Leone colony, that had been established by Granville Sharp in 1788, to provide a home and work for freed slaves. He even travelled as a passenger on a slave ship, so that he could understand what it was like to sail the Middle Passage.
When he returned to Britain in 1799, his ability at collating information and his good head for figures made him invaluable when it came to dealing with the huge amounts of evidence collected. He was also adept at drafting reports that highlighted the horrors of slavery.
He continued to work hard for abolition and, in the 1820s, focused on securing the total abolition of slavery in the British Colonies. He helped to set up the Society for the Mitigation and Gradual Abolition of Slavery (later known as the Anti-Slavery Society) in 1823 and edited the society's publication, Anti-Slavery Reporter. His hard work and clear arguments helped to pave the way for the 1833 act that saw the end of slavery in the British Empire. He died in London in 1838.