Dassanayake was born on 13 August 1942 in Katukurunda, Moratuwa as the second of the family. His father Don Dharmapala Dassanayake was a businessman. His mother Malalage Podi Nona Peiris was a teacher. He was educated at Anuruddha Vidyalaya, Nawalapitiya where his mother worked as a teacher and then attended to Nawandama Vidyalaya, Ja-Ela, Shiksadana Vidyalaya, Laksapathi and Hunumulla Central College. Finally he completed education from Horana Buddhist School (currently known as Don Pedrick Maha Vidyalaya Horana). Even though he was not good at studies, he surprisingly passed the Senior Examination in 1957. He has four siblings.
After interrupting school life, he once trained as a mechanic in a garage and as a radio reporter. Then he learned the log and typing. Afterwards, he just applied for the public health inspector exam. Having passed the preliminary inquiry, he went to the Treasury, which is near Parliament, but did not get the job.
He was married to Preethi Neela Wijesundara. He first met Neela during music classes at Government Music College known as Heywood College. Neela studied at Kadugannawa Central College and studied music at the Kandy Music Class. After that, she joined the Government College of Music and followed a six-year course. She learned esraj at the class. After the graduation, Dassanayake worked as a music teacher at Bibila Central College. Dassanayake worked as a music director and Music Inspector in Anuradhapura, Polonnaruwa, Vavuniya, Trincomalee, Jaffna districts in 1966. They married in December 1970. In 1977, he was appointed as the Deputy Director of Aesthetic Education in the Colombo District, while his wife worked as a music instructor at Lumbini Vidyalaya.
The couple has two sons - Ranga and Sanka - and one daughter, Lakmali. Elder son Ranga is a lecturer in sound administration at the University of the Visual and Performing Arts and also a film music director. Daughter Lakmali obtained a degree in English from the University of Delhi. Youngest son Sankha is a talented musician but mentally retarded.
Dassanayake died on 18 November 1999 at the age of 57 following a brief illness.