Mohanlal Viswanathan was born in the village of Elanthoor in the Pathanamthitta district on 21 May 1960. He is the youngest child of Viswanathan Nair, a former bureaucrat and law secretary with the Kerala Government, and Santhakumari. He grew up at Mudavanmugal, Poojappura, at his paternal house in Thiruvananthapuram. Mohanlal studied at Government Model Boys Higher Secondary School, Thiruvananthapuram and graduated with a Bachelor of Commerce degree from Mahatma Gandhi College in Thiruvananthapuram.

Mohanlal made his acting debut in 1978 with the film Thiranottam, which was produced and made by Mohanlal and his friends--Maniyan Pilla Raju, Suresh Kumar, Unni, Priyadarshan, Ravi Kumar and a few others. Mohanlal played Kuttappan, a mentally disabled servant. Due to some issues with censorship, the film was not released on time. It took 25 years to release the film.  In 1980, Mohanlal was cast in the lead antagonist role in Manjil Virinja Pookkal - the directorial debut of Fazil. The film became a major success. Mohanlal's friends had sent his application in response to an advertisement released by Navodaya Studio. He auditioned for the role in front of a panel that included professional directors. Displeased with his appearance, two of them gave him poor marks, but Fazil and Jijo Appachan gave him 90 and 95 marks out of 100. In an interview with Reader's Digest in 2004, Mohanlal said that his looks as a young man might have fit the villain's image.  By 1983, Mohanlal was credited in more than 25 feature films, most of them had him playing negative (villain) roles. Films such as Ente Mohangal Poovaninju, Iniyengilum, Visa, Attakkalasham, Kaliyil Alpam Karyam, Ente Mamattukkuttiyammakku, Engane Nee Marakkum, Unaru and Sreekrishna Parunthu changed his image. Through Sasikumar's Ivide Thudangunnu, he became a successful hero with a "good heart". Mohanlal played his first comic lead role in an ensemble cast in the 1984 comedy Poochakkoru Mookkuthi, directed by Priyadarshan, as a young man in love with a girl whom he mistakenly believes to be rich. It also marked the beginning of the Mohanlal-Priyadarshan duo, who as of 2016, have worked together in 44 films.  In 1985, he recorded a song for the film Onnanam Kunnil Oradi Kunnil. Uyarangalil, Nokketha Doorathu Kannum Nattu, Boeing Boeing and Aram + Aram = Kinnaram were some of his films in this period.

Did he have another acting role after Thiranottam?
lead antagonist role in Manjil Virinja Pookkal - the directorial debut of Fazil.