I don't know if this has ever been noted anywhere but just out of curiosity I went through my library's holdings of Who's Who to check which year was the first with an entry for Tolkien. I must say I was a bit surprised to find out that this was as early as 1925. I quote the entry in entirety just to show how it looked
"TOLKIEN, John Ronald Reuel, Professor of the English Language, University of Leeds, since 1924; b. 3 Jan. 1892; e. s.of late Arthur Reuel Tolkien, of Bloemfontein, South Africa; m. 1916, Edith, d. of late E. Bratt; two s. Educ.: King Edward VI. School, Birmingham; Exeter College, Oxford (Stapeldon Exhibitioner). First Class, English Language and Literature, 1915; served with Lancashire Fusiliers, 1915-18; Tutor in English School, Oxford, 1919; Reader in English Language, University of Leeds, 1920; Examiner in the Universities of Oxford and
Leeds. Publications: A Middle-English Vocabulary, 1922; Sir Gawain and the Green Knight (edited with E. V. Gordon), 1924. Recreations: tennis, fives. Address: 2 Darnley Road, West Park, Leeds." (Who''s Who, 1925, p 2855).
A possible conclusion from this might be that in 1925 there where few enough professors that a professorship in itself might be enough to render an entry in Who's Who. Also of slight interest to note is that the Tolkien/Gordon edition was listed despite not yet being published when this edition appeared as can be surmised from the year given (1924 instead of 1925).
Just in case you wonder: no entry for E. V. Gordon in this edition of Who's Who.