Karunas was born as Karunanidhi Sethu in Peravurani, Thanjavur, Tamil Nadu, India. He did his schooling at Palangudi Bharathi School and later studied at Nandanam Arts, Presidency College. Karunas began his career as pop singer and dancer, working extensively on Tamil folk music. He then gradually appeared in films as a comedian.
Karunas is married to Grace, a playback singer in the Tamil film industry. The pair had met when Karunas was the guest judge at an inter-collegiate singing competition, and after being impressed, Karunas asked her to sing in his personal music album.