Jennifer Jahner's research spans the long histories of rhetoric, law, and natural philosophy, focusing, in particular, on the European Middle Ages and its role in shaping our still-evolving understanding of rights, evidence, perception, and proof. Her research and teaching interests also include manuscript studies, visual culture, gender and sexuality studies, and history of science.

Jahner's first book, Literature and Law in the Era of Magna Carta (Oxford University Press, 2019) examines the grammatical and rhetorical training that informed constitutional thought in England at the beginnings of statutory law. It explores how poetry, which stood at the foundations of literate education in the Middle Ages, offered to law and its practitioners a powerful set of tools for conjuring theories of community, belonging, obligation, and injury. The book shows how the theory and practice of constitutionalism, articulated in legal codes and literature alike, emerged in tandem with the anti-Jewish laws and practices that would culminate in the 1290 expulsion of Jews from England.

Her current book project, Arts of Conjecture: Experimental and Literary Method in Later Medieval England, traces the early history of the experiment and its role in shaping vernacular literatures across the thirteenth, fourteenth, and fifteenth centuries. The concept of the "experiment" in this period amalgamated practices from a wide variety of fields, from medicine, to magic, to science, to visionary experience. The project explores the place of experiments in literary manuscript production and suggests that poetry offered another means of testing the boundaries of experiential observation and prediction.

With Emily Steiner and Elizabeth Tyler, Jahner co-edited Medieval Historical Writing: Britain and Ireland, 500–1500 (Cambridge University Press, 2019). She is also co-editor, with Ingrid Nelson, of Gender, Poetry, and the Form of Thought: Essays in Honor of Elizabeth A. Robertson (Lehigh University Press, forthcoming 2022). At Caltech, Jahner teaches courses on the history of gender of sexuality, Chaucer, romance, contemporary poetry and justice, the literature the marvelous and monstrous. She organizes the Literary Dimensions lecture series and co-organizes the James Michelin Distinguished Visitor (Writer-in-Residence) program and the Critical Intersections: Conversations on History, Race, and Science seminar series. She also serves as co-chair of the HSS Diversity, Equity, and Inclusion Committee and welcomes questions, concerns, feedback, and suggestions on ways to strengthen Caltech's commitment to, and practice of, equity and justice.