J.R.R. Tolkien held his first academic professorship in Leeds. In 1924 he became Professor of English Language at the University of Leeds, after serving there as a reader.
Tolkien’s Leeds years were important to his scholarly career and to his developing interest in invented languages and legend-making. He worked with E. V. Gordon on the 1925 edition of Sir Gawain and the Green Knight, a major academic edition that helped establish his reputation as a philologist.
Many people associate Tolkien chiefly with Oxford because he spent much of his academic life there. He became Rawlinson and Bosworth Professor of Anglo-Saxon at Oxford in 1925, moving from Leeds in the same year. Later, he held the Merton Professorship of English Language and Literature at Oxford until his retirement in 1959.
Leeds is therefore the answer to the question about his first professorship, while Oxford is the answer to where he became especially prominent as a professor and writer. The distinction is a common biographical mix-up.