Born on 6 May 1947 in Maharagama as the third in a family with seven siblings, he has two elder brothers, two younger brothers and two younger sisters. His father was Ilukpitiya Mudiyanselage Simon Singho who worked in the food department. His mother, Don Caroline Mabel Senadheera, worked at the Department of Health and later at the Colombo General Hospital. Nandasiri attended the Vidyakara Vidyalaya in Maharagama. He had an ambition to join military, because he was a sub-lieutenant in the school cadets. Nandasiri was briefly a catering officer at the Katunayake Airport in the late-'60s as his acting career developed.