Mohan was introduced to the world of theatre by B. V. Karanth, who spotted him at a restaurant. Mohan's first stage play was praised by critics from places like Delhi. Mohan was introduced to the cinema in Kannada by Balu Mahendra in his movie Kokila in 1977 alongside Tamil actor Kamal Haasan. Kokila was a hit and Mohan was catapulted into the limelight; since release of Moodu Pani in 1980 he has become one of the biggest stars of the Tamil cinema industry. He is called the "Silver Jubilee Star" and "Rajendra Kumar" of Tamil film industry since almost all movies featuring Mohan witnessed a fantastic run.
After Kokila, Mohan starred in a Malayalam movie titled Mathalasa. Furthermore, immediately after the success of the Malayalam movie, Mohan went on to sign a Telugu film titled Thoorpu Velle Railu, which was a remake of the Tamil film Kizhakkae Pogum Rail. The Telugu version was directed by Bapu. After that Mohan was introduced in the Tamil film Nenjathai Killathey by director Mahendran. In 1980 he acted in dual roles for the first time in Gandar Golam which was directed by director Singeetham Srinivasa Rao. Nenjathai Killaadhey marked the beginning of the peak of Mohan's career. Kilinjalgal, Payanagal Mudivathillai had Silver Jubilee. Mohan became a major star through Payanangal Mudivathillai (1982).