She is interested in eighteenth- and nineteenth-century British and American fiction, empire and sexuality, narrative and critical theory, visual culture, and scientific discourses at work in literary forms.
Armstrong's most influential book is Desire and Domestic Fiction: A Political History of the Novel (Oxford University Press, 1987), a work of scholarship still relevant thirty years after its publication.
As one reviewer put it, the book "changed the ways in which feminist critics of these novels saw the work these texts did; it changed the way we thought about public and private, agency and oppression, writing and action, giving us a far broader sense of the cultural work that novels do, as they translate political information into narratives about sex, gender, and desire.
In 1992, Armstrong published, together with Leonard Tennenhouse, "a pioneer[ing] study in the field of transatlantic literary relations", The Imaginary Puritan: Literature, Intellectual Labor, and the Origins of Personal Life (University of California Press, 1992), which looks at the relationship between the author and the emerging nation-state.
She is also managing editor of Novel: A Forum on Fiction and co-editor of Encyclopedia of British Literary History.