J.R.R. Tolkien served as Merton Professor of English Language and Literature at Merton College.
Tolkien held the Merton Professorship at the University of Oxford from 1945 until his retirement in 1959. Merton College, founded in the thirteenth century, was therefore the Oxford college associated with his final major academic chair.
Before that appointment, Tolkien had been Rawlinson and Bosworth Professor of Anglo-Saxon at Oxford and had also taught at the University of Leeds. His academic career focused on language, medieval literature, philology, and texts such as Beowulf. These subjects deeply informed the invented histories and languages of Middle-earth.
Tolkien’s university affiliations are sometimes mixed up because he studied at Exeter College and spent much of his career within the wider University of Oxford. The college named in this question is Merton, the institution connected with his professorship rather than his undergraduate studies.