Mary Mac is the nickname of a female Orion who by 2408 is employed as a social scientist on Forever World studying the Guardian of Forever. Her true name is too difficult to pronounce, although her surname begins with a sound approximating "Mac."
In order to be seen as less attractive, Mary Mac wore loose-fitted clothing, braided her hair, and wore glasses. She has Priority Alpha clearance to lower the force field which prevents anything from approaching the Guardian. Mar Loc, a renegade scientist also working on Forever World, used a hypo to knock out the sleeping Mary Mac so he could use her to access the Guardian and travel back to 2368 where he was able to assassinate Deanna Troi, thereby changing history.
In 2408 in the alternate future created by Troi's death, Admiral William T. Riker traveled to the Forever World and captured Mary Mac, using her to access the Guardian and travel back in time to prevent Troi's death thereby preventing the alteration in history from ever occurring. (TNG novel: Imzadi)