Born on 6 May 1944 in Maharagama as the third in a family with seven siblings, he has two elder brothers, two younger brothers and two younger sisters.[6] 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.[6] 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.

 

Nandasiri married fellow actress Devika Mihirani in 1989.She holds the record to be the main actress in Sri Lanka's first ever tele-drama, Dimuthu Muthu. Their first encounter was at Sudarshi Visual Arts Center, where Mihirani studied for dancing. Nandasiri was the one who invited Mihirani for the drama Subha Saha Yasa in 1974. She was studying at Anula Vidyalaya, Nugegoda. He joined her at the Chitrasena Kalaya in Kollupitiya and learned dance from Vajira. Devika's sister Sriyani Perera is married to actor Lal Kularatne.

 

They have two children, Navanjana and Rasanjana Suchitra. His daughter lives in Australia and son in America. Navanjana was born on 13 November 1989. Navanjana is married to businessman Chaminda Gunaratne and they married on 28 December 2012 in Hilton Colombo. His son Rasanjana started to continue his father's legacy by making mark in stage dramas, where his father previously acted.

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