Our best source on her work are the writings of Zosimos of Panopolis, who in many cases apparently simply quotes her wholesale rather than adding any information of his own. Though his books are the oldest known works on alchemy, the information that forms the basis of the tradition is attributed to her. The development of the basic processes of whitening and yellowing are described as being her work, as are several over-arching processes and principles of alchemy. According to her, “Join the male and the female and you will find what you seek; without the association of this union, nothing can be done right.”
She is best known for inventing or discovering (or at least being the first person to describe) certain instruments and materials. The most widely known of these is the bain-marie, a device that allows one to gently heat a substance to the boiling point of a given liquid (usually water) and no higher.* She is also credited with the tribikos (used in distillation), the kerotakis (an airtight container used to heat and collect vapors), and the discovery of hydrochloric acid.**
Regardless of her historicity, it is certainly interesting that a woman was widely credited as the first alchemist. The attribution of several inventions to her, including one still used today only adds to the intrigue.
*It’s essentially an early version of a double boiler.
**This last, at least, is almost certainly not true.
Women alchemists: Mary the Jewess and Cleopatra. Egypt, 2nd century AD? (Berthelot, Coll II 102) - found no. 424 in Lefkowitz and Fant, Women's Life in Greece and Rome
Patai, Raphael. The Jewish Alchemists: A History and Source Book. Princeton University Press, 1994. [found here on Google books]
Mary the Jewess - Wikipedia