Senanayake enlisted in the Sri Lanka Army through its 16th Officer Cadet Intake on 16 October 1981. Subsequent to completing his training, Senanayake was commissioned as a Second Lieutenant in the Corps of Engineers, joining the 1st Plant Engineer Regiment on 23 June 1983. He would go on to become a Troop Commander/Squadron Commander in the same regiment, as well as being posted as an instructor at Diyatalawa. In 1989, he took an inter-unit transfer to the Special Forces Regiment, where he served as Squadron Commander of the 1st Regiment Special Forces, and later commanded the 3rd Regiment Special Forces.
Higher command
Staff appointments he has undertaken include postings at the office of the Colonel - General Staff, 52nd Division; Brigadier - General Staff at Security Force Headquarters- Jaffna ; and Directing Staff at the Defence Services Command and Staff College, Sapugaskanda. In 2006, having attained the rank of Brigadier, he was appointed Director Plans at Army Headquarters, going on to work with the US Army as Senior Manager, Project Management of Afghan Operations and Strategic Planning for Civil Reserved Air Fleet. Senanyake has also held the positions of Regimental Center Commandant of the Special Forces Regiment, Brigade Commander of the 211 Infantry Brigade: Vavuniya, Commander- Special Forces Brigade, and General Officer Commanding the 52 Division in Varani, Jaffna.
Reinstatement
Following the results of the 2010 presidential elections, in which his former commandant Sarath Fonseka failed to defeat incumbent Mahinda Rajapakse, Senanayake was among Fonseka's loyalists in the military to flee the country, fearing reprisals from the Rajapakses. He returned to Sri Lanka following President Maithripala Sirisena's victory in the 2015 presidential election, was reinstated and appointed Military Secretary at Sri Lanka Army headquarters in February of that year. In 2016, he was appointed Commander, SFHQ-J, during which tenure he was responsible for overseeing the ongoing resettlement of Internally Displaced Persons from the Civil War in the Jaffna peninsula.
General Senanayake started active politics after his retirement from the position of Army Commander. He contested for 2019 Presidential election from National Peoples Party. He assured that he will lead professionals and intellectuals to parliamentary elections in 2020.
Personal life
Senanayake is married, and has two daughters and a son.