His first book was Literature as Opera (Oxford, 1977), a study of operatic versions of a wide variety of literary masterpieces; later, in 1990, Oxford also published his Shakespeare and Opera. In 1980 the University of California Press published his Shakespeare and the Courtly Aesthetic, which explored the influences of the aesthetics of the court of James I on the romances, focusing particularly on The Tempest. In 1990 the University Press of Kentucky published his Shakespeare and the Poet's Life, the writing of which was sponsored by a Guggenheim Foundation Fellowship. This study explored a central biographical question: why did Shakespeare choose to cease writing sonnets and court-focused long poems like The Rape of Lucrece and Venus and Adonis and continue writing plays?
In the 1990s, Schmidgall turned to biography -- gay biography in particular. The Stranger Wilde: Interpreting Oscar appeared in 1994 (Penguin), and in 1997 his Walt Whitman: A Gay Life appeared, also from Penguin (it won the Modern Language Association Prize for independent scholars in 1998). Work on Whitman led him to see the need for an edition of Leaves of Grass that offered a chronological selection of its poems in their first published form; this appeared from St. Martin's Press in 1999. In 2001, his edition of selections from the nine volumes of private conversations of Whitman transcribed by Horace Traubel, Intimate with Walt, appeared as the inaugural title in the "Iowa Whitman Series" of the University of Iowa Press. In 2006 Iowa University Press also published his edition, Conserving Walt Whitman's Fame: Selections from Horace Traubel's Conservator, 1890-1919. He is currently working on a book exploring Walt Whitman's ties to major figures in the English-speaking literary pantheon, among them Shakespeare, Milton, Blake, Burns, and Coleridge.
For nearly ten years beginning in 1992, Schmidgall served as the founding co-editor, later editor, of CUNY Matters, the news magazine for the City University of New York. Since 2003 he has contributd a bimonthly "Book Matters" column to CUNY Matters, featuring books authored by CUNY faculty members. Among his other projects are translations from the Latin of Martial's epigrams, an original play, an original opera libretto, and a monograph on Whitman and Allen Ginsberg.