J.R.R. Tolkien served as a professor at the University of Leeds in Leeds.
Tolkien joined the University of Leeds in 1920 as a reader in English language and later became professor of English language. His Leeds years were important in the development of his academic career and his collaborative work on language and medieval literature. He worked with E. V. Gordon on an influential edition of Sir Gawain and the Green Knight, published in 1925.
In 1925, Tolkien left Leeds for the Rawlinson and Bosworth Professorship of Anglo-Saxon at the University of Oxford. He later held the Merton Professorship of English Language and Literature at Oxford. These Oxford appointments are better known, which is why Leeds is sometimes overlooked in short biographies.
The city and university therefore form a distinct stage in Tolkien’s career between his early academic work and his long Oxford tenure. The question asks for the city, while the institution was the University of Leeds; both refer to the same period in his professional life.