Hoernlé spent nearly his entire working life engaged in the study of Indo-Aryan languages. His first paper appears in 1872, on the comparative grammar of Gauri languages. In 1878, he published a book on the comparative grammar of north Indian languages, which established his reputation as an insightful philologist as well as won him the Volney Prize of the Institut de France. During the 1880s, he published numerous notes and articles on numismatics and epigraphy in the "Journal of the Asiatic Society of Bengal" and the "Indian Antiquary", along with translations of medieval era Hindu and Jain Sanskrit texts.