Senathirajah was born on 27 October 1942.[1][2] He was educated Veemanramam School and Nadeswara College.[2] After school he joined the University of Ceylon, Peradeniya as an external student and graduated with a bachelor's degree.[2]
Senathirajah got involved in the Sri Lankan Tamil nationalism movement at a young age and took part in the 1961 satyagraha.[2] He joined the youth wing of Illankai Tamil Arasu Kachchi (ITAK), the Tamil Youth League, in 1962.[2] He was secretary of the Eela Thamil Elanger Eyakam (Eelam Tamil Youth Movement) from 1966 to 1969.[2] He was arrested on several occasions between 1969 and 1983 and spent seven years imprisoned at eight different prisons.[2] He became secretary of the Tamil Youth Front, the youth wing the Tamil United Liberation Front (TULF), in 1972.[2]
Senathirajah was general-secretary of ITAK before being elected leader of ITAK 6 September 2014.[16][17] Senathirajah is one of a trio of MPs (the other two being R. Sampanthan and M. A. Sumanthiran) who lead the TNA.[18][19]