Haynes was born on July 18, 1753, in West Hartford, Connecticut. He was the son of an African American man and a white woman. Haynes spent most of his childhood as an indentured servant in the house of a Granville, Massachusetts farmer.
As a young boy, he was drawn to church where he frequently participated. He began preaching while still in his youth. Haynes served in the militia during the American Revolution and also became an anti-slavery activist. He often argued against the involuntary servitude and preached against the slave trade.
Haynes became an ordained preacher in the Congregational church in 1785. Haynes pastored a church in Torrington, Connecticut for three years. In 1788, Haynes left the temporary position at the Torrington congregation to accept a call to pastor the West Parish Church of Rutland, Vermont, where he remained for the next 33 years. Lemuel Haynes died on September 28, 1833, at the age of 80.