Mary is a cheeky girl,with tow-coloured hair and light blue eyes which the Meredith children call 'white eyes'. Found by the Meredith children in a barn in the book Rainbow Valley, they take her home and look after her. She is later adopted by Miss Cornelia.  She had previously lived with a horrid Mrs. Wiley who whipped her until she ran away. Before that, her own parents were not extremely kind to her, and in the end they both died. The Manse and Blythe children don't have a terribly strong liking for her, but they 'can't help being nice to her'. They are very much in awe of her on accord to her boasting and swearing frequently.

In Rilla of Ingleside, Mary is having a romance with Miller Douglas, who later loses a leg in the war. In the end, she is engaged to him and frankly happy about it, though at the beginning Miss Cornelia does not approve.

In Rainbow Valley, it mentions that her full name is "Mary Martha Lucilla Moore Ball Vance".