Mallawaarachchi enlisted into the
Sri Lankan Army in 1971, upon his completion of his Officer Cadet training at the
Sri Lanka Military Academy,
Diyatalawa. He was commissioned a
Second Lieutenant of
Sri Lanka Light Infantry in 1972. He has had extensive training in many countries including the
United States and
India. He was a national champion in practical pistol shooting and is an excellent marksman. Major General Mallawaarachchi served the Sri Lankan Army for over 36 years throughout the island. He has held numerous positions in the Army, among them were The Acting Commander of the Sri Lankan Army, Chief of Staff of the Sri Lankan Army, Regimental commander of the Sri Lanka Light Infantry, 1st ever Security Forces Commander of Eastern area of Operations, General officer Commanding in
52 Division Jaffna, General officer Commanding in
23 Division Minneriya and Commandant and Vice Chancellor of the Kotawala Defence University. ==Diplomatic career==