Lal's first film was "Thiranottam" (1978). The film ran into trouble with the Censor Board and was never released. His breakthrough came in 1980, when he was selected for the role of the antagonist, in Manjil Virinja Pookkal, which was a blockbuster hit. He played roles of gradually increasing importance in a number of movies in the following years. In the year 1983, he was credited in more than 25 feature films. Uyarangalil, a story of cheating and deception, written by noted script-writer M.T.Vasudevan Nair and directed by I. V. Sasi was the highlight of this period. He then expanded into comic roles in his director-friend and college mate Priyadarshan's debut Poochakkoru Mookkuthi.
The period (1986 - 1995) is widely regarded as the golden age of Malayalam cinema with films characterised by detailed screenplays, lucid narration and fresh ideas narrowing the gap between art and commercial films. As a young talent on the rise, Mohanlal obtained roles that gave him ample scope to display a wide range of emotions and started some very fruitful associations with many of the better directors and writers in Malayalam cinema.
1986 was one of his best years. Sathyan Anthikad's T.P.Balagopalan M.A got him his first Kerala State award for best actor. His role of an underworld don in Rajavinte Makan saw the emergence of Mohanlal as the superstar of Malayalam cinema. In the same year, he played an asylum inmate in Thalavattom, a harassed house-owner in Sanmanassu Ullavarku Samadhanam, a journalist in M.T.Vasudevan Nair's Panchagni, a farm owner in love in Namukku Parkkan Munthiri Thoppukal and an unemployed youth forced to become a Goorkha in Gandhi Nagar 2nd street.
His association with the writer-director combination of Sreenivasan and Sathyan Anthikkad, who excelled in making socially hard-hitting satires, resulted in films such as Nadodikkattu, in which he played an unemployed youth and Varavelpu, in which he played a Gulf returnee who is welcomed back home to greedy relatives and a state with a hostile climate for entrepreneurs. Director Priyadarshan's musical comedies, notably Chithram and Kilukkam had him playing the typical Indian romantic hero and increased his popularity among the teenage movie-goers.
At his peak, he portrayed some of the best roles in Malayalam cinema in movies by Padmarajan and Bharathan. Thoovanathumbikal in which he portrayed a person torn between his twin love interests, broke many stereotypes in Indian films such as the leading man falling in love with a second woman immediately after he is rejected by the first. Amrithamgamaya was about a man who ends up at the house of a boy whom he had unknowingly killed in college during a ragging session. Thazhvaram was another notable film of this period.
The combination of writer Lohithadas and director Sibi Malayil is considered to have produced some of his most haunting roles. His role of Sethu Madhavan, who dreams of becoming a police officer but ends up as a criminal in the film Kireedam earned him a Special Jury award. Another role of a classical singer who is burdened by the jealousy and final death of his singer brother in Bharatham helped him bag the Best Actor National Award the following year.
The 90s saw him continuing his success with more entertainers like His Highness Abdullah, where he played a Muslim disguised as a Namboodiri to assassinate a royal. His other notable commercial movies during this period such as Midhunam, Minnaram, Thenmavin kombath continued the tradition of the 80s with neat well-written scripts and a rich set of characters. Devaasuram, written by Ranjith and directed by I. V. Sasi, set in the feudal atmosphere of central Kerala, was particularly noted for Mohanlal's portrayal of a proud, rich and brash young man who is slowly humbled by a chain of events. Director Bhadran's Spadikam became a cult classic for the revolutionary way in which the stunts were picturised. Critically acclaimed films were few in number during this period and were mostly semi-entertainers like Manichitrathazhu, which had the female lead Shobhana bagging a National award for best actress.
During this period of his career, film makers cashed in on the immense popularity that Mohanlal enjoyed in Kerala by portraying him as an invincible larger-than-life hero in movies. Movies like Aaram Thamburan, Narasimham, Ravanaprabhu, Naran used this image of his to great effect and became blockbusters. After their initial novelty, these films faced criticism from many quarters over their lack of realism and for creating movies around Mohanlal. Priyadarshan's Kalapani, an account of India's freedom struggle against the British that focused on the Cellular prisons of Andaman islands and Lohithadas's Kanmadam were some of his notable films during the late 90's.
It was at this time that Mohanlal started getting noticed outside the Malayalam speaking world. He acted in his first non-Malayalam movie when popular director Mani Ratnam roped him in for the Tamil film Iruvar. It had Mohanlal playing MGR, a cult figure in the neighbouring Tamil Nadu. The Indo-French production, Vaanaprastham, in which he played the role of a Kathakali dance artist with an identity crisis, won him the second National award for Best actor and was the first film that got him international recognition. The film was selected for the competitive section at the Cannes film festival and his performance was critically acclaimed.
In 2002, Mohanlal acted in his first Bollywood movie, Company, which introduced him to the wider Hindi speaking audience in India. It was a critical and commercial hit. During that time his career has declined for a short while in malayalam. In 2006, the film Thanmathra ("Molecule"), won him the Kerala state award for best actor for portraying a person affected by Alzheimer's disease. His second Bollywood movie Ram Gopal Varma Ki Aag, was the remake of the 1975 blockbuster movie Sholay, in which he played the character of the inspector played by Sanjeev Kumar in the original. Mohanlal won the 2007 Kerala state award for Best Actor for his performance as Valiakathu Moosa in the movie Paradesi. In 2009 Mohanlal acted with Dr.Kamal Hassan (Legendary actor and director) in Tamil movie Unnaipol Oruvan and praised by Tamil audience for his performance.