He graduated in 1938 and then studied at the University of Cambridge in 1938–1939, taking post-graduate courses in medieval literature and textual criticism.
Shortly after his return to Finland, Mustanoja's career as a teacher, academic and writer was interrupted by the Second World War.
His doctoral dissertation, The Good Wife Taught Her Daughter, an edition of three Late Middle English poems, was accepted at the University of Helsinki in 1948 with the highest grade.
It combines thorough familiarity with earlier research and profound analysis of the structural features of the language and a sensitivity to the finest nuances of meaning.
A Middle English Syntax, with an introduction by Elly van Gelderen, is available through John Benjamins Publishing Company.
He was a long time Editor of the society's publications, and during his editorship Neuphilologische Mitteilungen established its position as an internationally recognised scholarly journal specialising in English, German and Romance language philologies.
For over three decades, Mustanoja was chairman of the board of the United States Educational Foundation in Finland (the Fulbright Program).
In this area, he published articles on a wide range of authors, including Chaucer, Shakespeare, Longfellow, Maugham, Franςois Villon and Henry James.
Tauno Mustanoja held the post of chair of English Philology at the University of Helsinki until his retirement in 1975; his students included Matti Kilpiö.
His last book, a collection of essays called Runo ja kulkuri ( 'The Poem and the Vagabond', 1981 ), contains a fine analysis of Western literature and civilisation ranging from classical antiquity through Medieval Europe to Renaissance England.