He joined the University of Calcutta as a lecturer in 1930, where he served as a professor for thirty four years. He became the second Khaira Professor in the Department of Comparative Philology after his mentor, Suniti Kumar Chatterji, in 1954. After assuming this title, the department attracted many scholars from India and abroad to study and conduct research. Sen was the first scholar to explore the Old Indo-Aryan syntax in his book, Use of Cases in Vedic Prose, and Buddhist Hybrid Sanskrit. He later analysed the syntax of Middle Indo-Aryan in An Outline of Syntax of Middle Indo-Aryan. He contributed significantly to Bengali literature, addressing themes ranging from mythology, the Puranas and crime to horror. Sen's crime stories were compiled in the book Galpa Samgraha. He published numerous significant articles and research papers. These include the Bangla Sahityer Itihas, Bhashar Itibritta, A History of Brajabuli Literature, A Comparative Grammar of Middle Indo-Aryan, Ramkathar Prak Itihas, Bangla Sthannaam, Bharat Kathar Granthimochan, Bharatiya Arya Sahityer Itihas and Women's Dialect in Bengali. Bhashar Itibritta is the first book in the Bengali language on Indo-Aryan and Indo-European historical linguistics. In this book, he postulated Jharkhandi as the fifth dialect of the Bengali language. His book Bangala Sahitye Gadya remains the best example of a systematic, stylistic description of the literary dialect of the language. The Etymological Dictionary of Bengali is one of the largest works on historical etymology in any Indian language. Bangala Sahityer Itihas was also a monumental contribution. Rabindranath Tagore commended the book and wrote the preface. The English edition was published by the Sahitya Akademi in 1960. Pandit Jawaharlal Nehru wrote the foreword for this book. His autobiography was 'Diner Pore Din Je Gelo'.
Recognition
The London Asiatic Society honoured him with a Jubilee Gold Medal in 1984, making him the first Asian to receive the prize. Other prizes include the Rabindra Puraskar, Ananda Puraskar, Vidyasagar Puraskar, Desikottam, and the Padma Bhushan. The Asiatic Society, Calcutta, awarded him the Jadunath Sarkar Medal. He was elected as an honorary fellow of Sahitya Akademi in 1973. He received the prestigious Ashutosh Memorial Gold Medal and Griffith Memorial prize twice. He was also awarded the University Gold medal and Sarojini medal. A college in Gotan was named in his honour.
