Sources

After some research I’m certain this is the same Mary Spenser/Spencer. In the biographical notes in The Female Prose Writers of America: With Portraits, Biographical Notices, and Specimens of their Writings it is mentioned she is married to an engraver from Philadelphia. With this information I found engraver Joseph Ives Pease, who was based in Philadelphia (see Pease family research on rootsweb), who married a Mary Spencer in the correct time frame.

She seems to have died really young, perhaps the reason it’s hard to find much biographical information on her. She does have a lot of contributions to various magazines that can be found on Google Books, as well as mentions of friendships in literary circles.