In December 1940, the Ceylon Trade Union Federation (CTUF) was formed and Mendis was one of the organisations leaders.
In September 1945, the CTUF led by Mendis organised a march of 10,000 workers to the State Council to present their grievances, which was followed by a six-day strike by tram workers during which Mendis and other trade union officials were arrested.
In 1947 he ran in the first parliamentary elections for the Communist Party in the Colombo South Electoral District but was unsuccessful, only gaining just over 10% of the vote.
[3] In 1965 he contested the 6th parliamentary election in the Rathgama Electoral District where he finished second by 1,210 votes.
[4] Mendis successfully ran again in 1970 at the 7th parliamentary election for Lanka Sama Samaja Party, with approximately 65% of the total vote.