Born in New York in 1775, John Marrant moved around a lot in his early childhood. He was a free black man. Eager to learn a trade, he starts inquiring, but quickly becomes side-tracked by a fascination with music. He learns to play the violin and the french horn, becoming rather consistently employed as a musician. He does, in fact learn a trade, and falls in with a rowdy crowd. One day he and some friends crash a church meeting, but John is struck by the message of preacher, George Whitehead. He actually converts right there and devotes his life to religion.
Meanwhile John's family essentially disowns him, claiming he has become unstable and insane. They were mostly speaking about his actions among his old crew of friends, but they distrust this conversion as much as any of his previous actions. On his own, John takes up with a Native American who takes him around American to preach for Cherokee tribes. He lives with them for some years.
When war breaks out, John is forced into the Royal Navy and serves as a musician. By the end of the war he is in London. He reunites there with George Whitehead and tells him what an impact his conversion has made. In turn, Whitehead puts John in contact with a wealth patroness, Lady Huntingdon. She helps John become ordained as a minister as well as publishing his life's story -- this book. John accepts a ministry position in Nova Scotia, upon the invitation of his brother.