[1] Born and raised in the village of Bakmeedeniya, Kegalle, in a family of six children, Ranatunga attended St. Sylvester's College, Kandy; where he excelled at many sports, and captained the hockey and athletic teams.

He joined the Ceylon Army in 1950 and received his officer training at Royal Military Academy Sandhurst, where his company commander was Patrick Anthony Porteous.

In 1962, he attended the Staff College, Camberley, where he became friends with Major Yakubu Gowon, later General and Head of the Federal Military Government of Nigeria.

Following the clearing of insurgents from Kegalle, he was appointed Military Coordinating Officer of the Anuradhapura District to lead mop-up operations.

In February 1982, he was appointed Chief of Staff of the Sri Lanka Army and Commander Security Forces Headquarters – Jaffna.

Sri Lanka faced restrictions for weapon imports from the west as the case of the US government approving the sale of Cadillac Gage armored cars without turrets and guns; as well as the British government's refusal to supply spare parts for the Alvis Saladins and Ferret armoured cars.

His tenure as GOC-JOC saw large scale offensives against the LTTE such as the Vadamarachchi Operation which came close to meeting its objective when it was abandoned due to Indian intervention when military cargo planes escorted by fighter aircraft dropped humanitarian relief supplies in the Jaffna area on June 4 in Operation Poomalai.

[1] On 1 August 1986, he was promoted to full general[2] and was retired from active service in 1988 once again, stepping down as GOC, Joint Operations Command.

However, four months into his tenure, Premadasa's successor Wijetunga appointed him as Sri Lankan High Commissioner to the United Kingdom.

[citation needed] Leaving public life, he lived the remainder of his days at his ancestral home in Mawanella having become a gentleman farmer.

He died on 16 June 2021, and per his family's wishes a private funeral took place at Borella General Cemetery on the following day.

[3] On his way to the British Army Armoured Center in Bovington on board P&O liner Orontes, he met Myrtle Sumanasekera, they were engaged on their arrival in London and married soon after.