Thornbrough was interested in women's history, but other than her book on Blaker and an essay she contributed on the history of Indiana’s black women, which appears in Indiana’s African-American Heritage (1993), her published scholarship was in other areas. Legal and constitutional history, as well as black history, were her major interests. Thornbrough's published works related to black history include Black Reconstructionists (1972), Since Emancipation: A Short History of Indiana Negroes, 1863–1963 ([1963?]), and topics such as the fugitive slave law in Indiana, the history of the civil rights movement in Indiana, and Indianapolis's school desegregation, among others.
