‘Amazing Grace’ was written by John Newton in 1772 and was first published in 1779 in a collection called ‘Olney Hymns’ in which all the lyrics were by either Newton or the poet William Cowper. We have no idea what tune was originally sung to this hymn.
‘Amazing Grace’ became very popular in 19th century America and it was there that it was first sung to the American folk tune we now associate with it.
Interestingly, Newton’s original final verse was different from the ‘When we’ve been there ten thousand years’ verse that we now sing. The substitution was made by an American editor, who replaced the original (which he apparently felt was too Calvinistic?) with the words now familiar to us. The difference is easily noticeable in that Newton’s original verses are all in the first person singular, while the new verse is in the plural.
I have written my own tune to ‘Amazing Grace’, and have chosen to sing Newton’s original words, not the later American edition.