Karthik was first introduced by Bharathiraja in the film Alaigal Oivathillai. He received the Best New Face Award from the Tamil Nadu Government in 1980. He collaborated with leading film directors like Maniratnam, Balachandar, Bharathiraja, R.Sundarrajan, R.V.Udayakumar, Priyadarshan, Fazil, Vikraman, Agathiyan, Sundar.C, KS.Ravikumar and P.Vasu.
He is a celebrated actor of Tamil Nadu and was awarded the title "Navarasa Nayagan" (multi-talented). He is popular with the audience for sense of humor and was involved with several small comedy scenes in most of his movies. His cameo in Mouna Ragam was very well received by the audience. During his later years, he formed a partnership with Goundamani in many comedy movies. His pairing with Rambha was a hit with audience as the pair provided good hits in the nineties.
In 2009, Karthik signed on to play a supporting role in Mani Ratnam's Raavanan, thus making a comeback in Tamil cinema. His first release of the innings was however in Maanja Velu, in which he played a police officer alongside Arun Vijay, though the film became a financial failure. He received critical acclaim for his portrayal of Gnanapragasam in Raavanan featuring Vikram, Aishwarya Rai and Prithviraj. He also went on to feature in a supporting role in Puli Vesham directed by P. Vasu, although the film went largely unnoticed.
Karthik entered politics ahead of the 2006 Tamil Nadu Legislative Assembly election. He joined the All India Forward Bloc, and was appointed as the Secretary of the Tamil Nadu state unit of the party. He led the party during its electoral campaign on 24 September 2006. He later started his own party Ahila India Naadalum Makkal Katchi in 2009 before the Lok Sabha Elections. He contested from Virudhunagar constituency. Allegedly he never campaigned and was routed in the election.