Karthik developed an interest in music, early in his life. He is a trained Carnatic music vocalist and has been training since his school days. He was part of a college band and regularly participated in IIT Madras Fest Saarang. An ardent fan of A. R. Rahman, Karthik’s dream was to meet Rahman and sing for him. Playback singer Srinivas, who collaborated often with Rahman was Karthik’s close friend’s cousin. Srinivas encouraged Karthik to look at singing as a career, which prompted Karthik to resume his singing lessons. One year later, Srinivas recommended Karthik’s name to Rahman, who needed fresh voices for backing vocalists for a song from Pukar. Karthik eventually got the opportunity and recorded for the song. Having spent over a year being a backing vocalist, Karthik got his break during the re-recording of One 2 Ka 4. For the background score, A. R. Rahman wanted a high-pitched alap and he eventually chose Karthik to sing the alap. Soon after, Rahman asked him to sing the song “Nendhukittaen” from the film Star.
Karthik has sung a number of songs for many popular music directors, including A. R. Rahman, Ilaiyaraaja, Vidyasagar, Yuvan Shankar Raja, Harris Jeyaraj and Manisharma. Karthik has sung songs in several languages – Hindi, Tamil, Telugu, Malayalam, and Kannada. “Ennaku Oru Girlfriend” from the Tamil film Boys was one of the biggest hits of the year as was “Oru Maalai” (Ghajini; 2005). His songs for Telugu films for Kotha Bangaru Lokam (Nijanga Nenenaa), Happy Days (Arare Arare, Oh My Friend), “Badhulu Tochanai” (Mr.Perfect), “Manase Guvvai” (Naa Peru Shiva), “Oh Oh My Friend” (Oh My Friend), “Meghama” (Journey), “Yevvaro” (Bodyguard (Telugu)) are very popular. His other popular songs include “Ava Enna” from the Vaaranam Aayiram (2009), the title track from Unnale Unnale, “Behka” from Ghajini and “Alle Alle” from the Kannada film Savaari.
In 2010, he sang a track “Behene De”, from Hindi film Raavan, which is widely considered as the biggest song in his career. The track, which toped music charts for many continuous weeks, also received rave critical reviews. Karthik also performed the Tamil and Telugu versions of the same song, titled “Usure Poghudhey” and “Usire Poyene” respectively. He also sang “Adhigora Choodu” in the movie Mirapakaay.