Is the song Amazing Grace about Mary?

No. The words to the song were written by Englishman John Newton, a slave trader, as a response to a powerful conversion experience. After a terrifying storm at sea, Newton realized that it was only by God’s grace that he would be saved; these sentiments later became the basis for the song, which was written in 1772. The Library of Congress website has a good explanation of the story behind this popular hymn.