In 2007, his investigative feature, "The Dark Legacy of Carlos Castaneda", revealed the story of the secretive cult started by the controversial anthropologist, and told the story of the five women who disappeared following his death in 1998. It was chosen for Best of Salon 2007. Soon after, Marshall began work on a comprehensive biography of Castaneda. His pursuit of this story led to a confrontation with law enforcement officials in Death Valley in 2014. In 2016, the Biographers International Organization awarded Marshall the Hazel Rowley Prize for this biography-in- progress. His criticism has appeared in Artnet, PAJ, the Lambda Literary Review, and Public Books. In 2017, Marshall organized Writers Resist Trump, a large-scale lobbying effort and demonstration that accompanied the AWP (Association of Writers and Writing Programs) Conference in Washington D.C. Marshall told "Publisher's Weekly:" "I looked and didn't see anything about people going to congress, and I thought that was a little weird." He then began talking to like-minded friends and started a Facebook group to plan and gather participants. Marshall began to exhibit in New York City in the early '90s. Since then his work has appeared at such venues as the Baxter Street Gallery, Richard Anderson Fine Arts, The Thread Waxing Space, Art in General, White Columns, Derek Eller Gallery, Triple Candie, Bronwyn Keenan Gallery, Steffany Martz Gallery, The Drawing Center, Dooley Le Cappellaine Gallery, Amy Lipton Gallery, Berland/Hall Gallery, Peter Kilchmann Galerie (Zurich), Centro Cultural Ricardo Rojas (Buenos Aires), 2B Gallery (Budapest), Gallery 32 (London), the New Orleans Contemporary Arts Center, The Other Side Gallery (Memphis), Art and Idea (Mexico City), Wessel O'Connor Gallery, and the John Tevis Project Space. He has also participated in panels at venues such as Dillon and Lee Gallery, Studio 10, and is also a guest artists for Yaddo. He is a recipient of fellowships from the MacDowell Colony, the Corporation of Yaddo, the Virginia Center for the Performing Arts, the Ragdale Foundation, the Banff Centre and the New York Foundation for the Arts. From 2002 to 2005, Marshall served as director of Prose in General, a reading series at Art in General focusing on the intersection of fiction and the visual arts. In 2004, he started teaching at the International Center of Photography in New York City.