Mallawarachchi was born on 7 April 1944 in Kotte, Colombo, Sri Lanka as the only child in the family. His father was Upasena Mallawarachchi and mother was Agnes Wickramarachchi. He lost his father at an early age, where he was a courageous child who struggled to win the battle of life with his mother in the midst of many hardships during his school days. He completed his education at Ananda Sastralaya, Kotte. After completing his education, he got his first job as a bill clerk at the Welikada Co-operative and later worked as a cashier at the Nawaloka business.

 

On December 23, 1973, he married Swarnamala Malathi Panangala. The couple had four children: Ranil, Harshini, Nadija, and Tharaka. The song, "Etha Epita Dura Akase", written by Karunarathne Abeysekara for Mallawarachchi, is dedicated to his wife Swarna.

 

Eldest son, Ranil Mallawarachchi, continues his father's legacy by performing his songs in shows and concerts. Ranil is married to Nirasha, a fashion blogger. The couple has one son and one daughter. Ranil's son, Pawan Mallawarachchi also entered music to his grandfather's legacy. Pawan is the youngest pilot to train as a pilot in Sri Lanka and Australia. Milton's younger son Tharaka is also a singer who sang the songs: Satuṭin Samugena, Virasaka Sitaṭa Lanvelā and A Dine Patan.

 

He died on 10 March 1998, at the age of 53, and is buried in the Borella cemetery.