After finishing education, Mendis along with fellow actor Senaka Perera started to produce a stage play Illeesa. In 1979, Sriyantha was introduced to the stage for the first time as the devil in the play Parassa by its producer Rohana Dandeniya. He also had the opportunity to act in Neville Dias Subasinghe's play Manawayo. In November 1979, he acted in another stage play Sathuro produced by doctor Chandrapala, who worked at Ragama rehabilitation hospital. Meanwhile, he wanted to study drama, and met German theater producer Norbert J. Myer. Then he played in Myer's comedy play Ane Abilik. In 1984, he acted in the teledrama Palingu Menike directed by Dhamma Jagoda. The character 'Surasena' was so popular that there were people who imitated the character, behavior and dialogues.
