Hamilton Wanasinghe was born in Malwana, a suburb of Colombo. He received his primary and secondary education at Ananda College. A keen sportsman, he was the Sergeant Major in the College Cadet Platoon. He was also an active member of the College rifle shooting team, which won many coveted trophies and then later, he joined the army. Wanasinghe also represented Ceylon Inter Dominion small bore rifle shooting competition.
He was appointed as the Army Commander on 15 August 1988 and served until 15 November 1991 and was promoted to the rank of Lieutenant General. When Wanasinghe took command the army was engaged in subduing 2nd JVP insurrection in the south of the island, while the Indian Peace Keeping Force was present in North and East of Sri Lanka. General Wanasinghe had the unprecedented task of maintaining cordial relations in a turbulent period. During his tenure he expanded the army and raised number of new units including special forces and established its sniper school, the army was able to crush the JVP leadership and insurrection completely, while it was able to recapture the Eastern Province and allow the elections to be held in the Eastern Province. The army carriedout several major operations in the Northern Province, including Operation Balavegaya to relive the siege of Elephant Pass Army base through an amphibious operation and established a beach head at Vettalikerni to reach Elephant Pass camp. Indian Defence Review in 1991 reported that "Army Commander Lt Gen Hamilton Wanasinghe visited the battlefield frequently and narrowly escaped being killed. " Lt.General Hamilton Wanasinghe as the Commander of the Army risked his own life and was present at Elephant Pass rescue operation Balawegaya along with Major General Denzil Kobbekaduwa and Task Force Commander Late Brigadier Vijaya Wimalaratne. Operation Balavegaya was launched in June 1991 with assistance of Sri Lanka Navy with an amphibious landing at Vetterlerkani which LTTE leader called it "Mother of All Battles". Victorious Sri Lanka Army troops broke the LTTE siege at Elephant Pass.
On relinquishment of the Commander of Army on 15 November 1991 he was promoted to the rank of General and appointed as the General Officer Commanding of the Joint Operations Headquarters serving from 1991 to 1993.
Thereafter he was appointed Permanent Secretary of the Ministry of Defence on 6 June 1993 from which position he retired in February 1995. He had a rare distinction of holding all three key appointments of the Sri Lanka's Defence establishment at a time country faced southern JVP insurrection and LTTE Second Ealam War. In November 1993 General Hamilton Wanasinghe facilitated the relief operation during the Battle of Pooneryn with an amphibious landing of Army troops assisted by the Navy.
He married Ira Beatrice Jayathillake daughter of Wanasinghe Arachchige Elaris Perera Jayathillake Ralahamy, a close relative in 1960. Ira Wanasinghe was a school teacher and their had five children. His son Sanjaya Wanasinghe was commissioned as a Second Lieutenant in the Sri Lanka Artillery when Wanasinghe was the Army Commander. Major General Sanjaya Wanasinghe commanded army snipers during last stage of the civil war as the Commanding Officer, Marksmanship and Sniper Training School, Diyatalawa. Hamilton Wanasinghe's lost two of his nephews in 1993 while in active service, Captain Nalin Jayatilleke in Jaffna and Major Panduka Wanasinghe in Pooneryn. His son-in-law Brigadier Bhathiya Jayatilleka was killed in 2000 during the Second Battle of Elephant Pass.
