Ganesh Kumar made his debut in the 1985 Malayalam film ‘Erakal’ directed by KG George. The film was adjudged the second best film for the year. Over the years he made his a mark on the silver screen as a supporting actor, performing a variety of roles during the last two decades. Since 2001 he has been balancing his role as MLA with his acting duties, indicating his unwillingness to give up his first love.
Ganesh Kumar’s early films saw him frequently donning the role of villain or anti-social. In Cheppu (1987) he played the rowdy college student to perfection, experimenting with brown sugar, leading an ruly pack, and taunting the noble professor (played by Mohanlal). Eventually the hero beats the daylights out of him, and after some ugly display of blood and tears, the movie comes to a predictable end. In Rakuyilin Ragasadassil, Ganesh Kumar played a wayward guitarist who turns over a new leaf after settling down with a nice girl. This is in stark contrast to the story of the lead pair (his brother and musical genius, played by Mammooty, and the talented danseuse, played by Suhasini) who part ways soon after the birth of their son, only to reunite in a tearful climax. Other films of the eighties where Ganesh made minor appearances include Sukhamodevi (1986), Sarvakalasala (1987), Bhoomiyile Rajakkanmaar (1987), Mukthi (1988), and Devadas (1989).
During the nineties, Ganesh Kumar played notable supporting roles in various films including Gajakesariyogam (1990), Maala Yogam (1990), Randaam Varavu (1990), Kilukkam (1991), Kuttapathram (1991), Njan Gandharavan (1991), Parallel College (1991), Mahanagaram (1992), Kaazhchakkappuram (1992), Ooty Pattanam (1992), Ammayana Sathyam (1993), Customs Diary (1993), Ekalavyan (1993), Vishnu (1994), Commissioner (1994), Rudraksham (1994), The King (1995), Aayiram Naavulla Ananthan (1996), Kalyanapittennu (1997), Varnapakittu (1997), Usthad (1999), Crime File (1999) and Aayiram Meni (1999).
The current decade saw Ganesh Kumar emerge as a significant player on the political scene after his election to the Kerala State Legislative Assembly in 2001, and his accession to the ministerial chair the same year, followed by his dramatic exit in 2003 to accommodate his father and party supremo, R Balakrishna Pillai. His subsequent re-election in 2006, overcoming a State-wide pro-LDF wave, saw him firmly established as a rising star on the political firmament of the State. All this did not deter him from relentlessly pursuing his acting career. However, he seems to have chosen his roles with some measure of caution. Somewhere along the way, Ganesh Kumar abandoned the garb of the villain and took up the mantle of the good Samaritan, perhaps reflecting his societal roles and obligations. He acted in Pilots and Dada Sahib in the year 2000. In Kilichundan Mampazham (2003) he played the heroine’s brother Ummer. In Vismayathumbathu (2004) Ganesh Kumar played a medical college professor. He also acted in Photographer (2006), Hello (2007), Chembada (2008) and Veruthe Oru Bharya (2008).
The film 'Lion' starring Dileep and Kavya Madhavan (released in 2005) was inspired by Ganesh Kumar's life. Krishna Kumar (played by Dileep) is the innocent son of an unscrupulous father, struggling to build a political base in the midst of deceit, intrigue and cut-throat competition. His father, quite aptly named Balagangadhara Menon, is a seasoned and corrupt politician, and he has two diabolic sons-in-law to assist him in his dark deeds. How the son grew up to be so virtuous is a total mystery! The film, despite the exaggerated and unrealistic story line was a thorough entertainer and did extremely well at the box office.
In March 2008, Ganesh Kumar was chosen by the Frame Media gallup poll to receive the best actor award for his role in the Amrita TV serial “Aliyanmarum Penganmarum”. He went on to win the State Award for Best Television Actor (2007) for his role in the serial "Madhavam" aired on Surya TV.