Sellasamy was district chairman of the
Ceylon Workers' Congress (CWC) before being elected its general-secretary in 1963. He was an executive member of the Colombo District Development Council from 1981 to 1988. He was appointed Minister of Health and Economic Infrastructure. He was appointed Minister of State for Transport on 18 February 1989. He became Minister of State for Industries on 30 March 1990. Sellasamy was removed as general-secretary of the CWC in 1994 and subsequently formed the Ceylon National Workers' Congress (CNWC). A long legal battle ensued between Sellasamy and CWC leader
Savumiamoorthy Thondaman which prevented the CWC from using its "Cockerel" symbol to contest elections. Following the death of Thondaman in 1999 Sellasamy tried unsuccessfully to gain the leadership of the CWC from Thondaman's grandson
Arumugam Thondaman. Sellasamy was appointed as one of the CNWC/
DWC/
UCPF/
UNP alliance's
National List MP's in the Sri Lankan Parliament following the
2000 parliamentary election. Sellasamy rejoined the CWC in October 2001 as its deputy president. He contested the
2001 parliamentary election as one of the
United National Front's (UNF) candidates in Colombo District but failed to get elected. He was appointed as one of the UNF's National List MP's in the Sri Lankan Parliament following the
2004 parliamentary election. He was appointed Deputy Minister of Posts in January 2007. Sellasamy was a member of the
University of Colombo's
senate and the National Agricultural Diversification and Settlement Authority (NADSA). ==Electoral history==