Kusuma Jayasooriya was born on November 21, 1940 at Dickwella, Dodampahala to a business family of Mr. and Mrs. D. E. Jayasooriya. She had her school education at Dodampahala Vidyalaya and then at Vijitha Vidyalaya, Dickwella. She completed her undergraduate education at the University of Ceylon in the Peradeniya campus; there in 1964, she became the first female student in her discipline to earn a First class honours degree from that institution. She later married Samarajeewa Karunaratne, an engineer by profession. They have two sons, Savant Kaushalya and Passant Vatsalya, both electrical engineers specializing in Image processing. The elder, Savant Karunaratne has a PhD in Electrical and Computer Engineering from the University of Sydney, Australia. The younger, Passant Karunaratne is a Principal Research Engineer in United States.
An academic career began with her appointment as an assistant lecturer of the Department of Sinhala at the University of Ceylon. In 1967, Mrs. Karunaratne and her husband traveled to the United Kingdom where both furthered their education with post-graduate studies, with her studying Sociology of Literature at the University of Essex. Later she pursued graduate studies at the School of Oriental and African Studies in London. She was awarded her doctorate from the University of Colombo.
Karunaratne became the first female lecturer in Sinhala Department at the University of Columbo, and she was to be the first female Professor of Sinhala and the first woman to head that Department. Professor Karunaratne would later go on to become the Dean and the Acting Vice-Chancellor of the University of Colombo.
During her career she had taught Sinhala to many Japanese, including professors and diplomats, thus helping to strengthen the relations between Japan and Sri Lanka. She had translated many English and Japanese novels to Sinhala.
- A Glimpse of Japanese Culture 
- Selected Sri Lankan Short Stories, Vol. I & Vol. II., co-authored (with Sarath Wijesooriya), published by Godage Publishing ISBN 955-20-6238-1, ISBN 955-20-6239-X
- English-Sinhalese translation of Hemingway's The Sun Also Rises
Honors and awards
- Ruhunu Putra Award
- Liya Waruna Award
- Fulbright, Fellow.
- Japan Foundation, Fellow.
- Order of the Rising Sun, Gold Rays and Neck Ribbon
- "Darling we aren't too old, though we pass the Jubilee Gold: Mathematician + intellectual = love,Sunday Observer, January 7, 2007.
- Tissera-Gunasekara, Manu. "Kusuma Karunaratne: The simple, traditional Sri Lankan lady," Daily Mirror. August 15, 2007.
- Perera, Suharshi. "Woman: An icon in the literary world," Daily News (Sri Lanka). August 9, 2007.
- Speech at Seminar in Commemoration of Fifty Years of Bilateral Relations between Sri Lanka and Japan," (2002).