Like many other writers before and after (including her own father), Mary Shelley took a shot at being a playwright. This does not usually work out well for people who are not especially talented at dramatization—to wit (always wanted to use that expression), Henry James or John O’Hara or Norman Mailer or numerous others who wrote fine novels or stories but failed on the stage. (James had the misfortune of mounting a play in London at the time Oscar Wilde was emerging—a man who could write powerfully in more than a genre or two.)
In the course of my research, by the way, I did read the plays written by her father, William Godwin. And—to be punny—they were God-lose. He was a fine novelist (at times), but his plays—St. Dunstan (1790—unpublished), Antonio: A Tragedy in Five Acts (1800), Abbas: King of Persia (1801—unpublished), and Faulkener: A Tragedy (1807) … tragedies in every sense. (I did not read the two unpublished ones, which, until 2010, existed only in manuscript. In 2010, they were all published, but the cost, for me, is prohibitive: $200.)
In the spring of 1820, living with Bysshe in Pisa and, while she was slowly researching and writing Valperga, she wrote two “unpublished mythological dramas,” as they were called when they first appeared in print more than a century later, 1922. This 102-year-delay gives some clue about their success. Mary did attempt to get them published, and in November 1831 she got a revised version of Proserpine published in an annual periodical (The Winter’s Wreath), but neither brief play ever appeared onstage during her lifetime.
I say “brief.” I see in my notes that I read them both on July 21, 1997—six months after I’d retired from my public school teaching career. My journal—which at the time was very perfunctory (to say the least)—does not say much. We were in the midst of trying to sell our wonderful house in Aurora, Ohio, for we had found a smaller place in nearby Hudson that we loved and where (as I type this on December 18, 2015) we are still living. In the morning, I read all of Proserpine at Saywell’s Drug Store (and soda fountain and coffee shop, R.I.P.), our favorite hangout at the time, and commented with only this in my journal: some moving lines and moments. That’s helpful.
Later in the day I read all of Midas but said only that I finished it and typed notes on the two plays even later on. Each two-act play got a single page, single-spaced, 11-pt. font. Appropriate, I fear.
 Proserpine & Midas: Two Unpublished Mythological Dramas, ed. A. Koszul (London: Humphrey Milford, 1922).