The son of Chandi Prasad Singh, an erstwhile Indian prince and a prominent criminal lawyer, K.N. Singh was a sportsman who once dreamt of being in the army. Born in Dehradun, Singh was expected to follow in the footsteps of his father and become a lawyer. However, his father's skillful defence, which saved an obviously guilty man from the gallows, turned him away from the profession.
Turning his energy to sports, K.N. Singh came to excel in Javelin throw and Shot Put. He was selected to represent India in the 1936 Berlin Olympics before circumstances compelled him to go to Calcutta to attend on his ailing sister. There he met his family friend Prithviraj Kapoor, who introduced him to director Debaki Bose, who offered him a debut role in his film Sunehra Sansar (1936).
K.N. Singh enjoyed limited success until the release of Baghban (1938), in which he played the antagonist. Baghban was a golden jubilee hit, establishing Singh as one of the leading villains of the era.
Through the 1940s and 1950s, Singh appeared in several iconic movies of the era, including Sikandar (1941), Jwar Bhata (1944) (Dilip Kumar's film debut), Humayun (1945), Awara (1951), Jaal (1952), CID (1956), Howrah Bridge (1958), Chalti Ka Naam Gaadi (1958), Amrapali (1966) and An Evening in Paris (1967).
As opposed to playing angry mobsters, he mostly played a white collared gentleman villain, dressed in a fine suit and smoking a pipe, with a calm cold delivery.
His suave style, baritone voice and menacing eyes became legendary so much so that on one occasion (in his own words) "Even off-screen I was a bad man. One day on my way back from shooting, I had to deliver an envelope at an address given to me by my friend. I pressed the doorbell and, from the moving curtains, I could see a woman hurrying to open the door. When she saw me standing in front of her, she screamed out in fright and ran inside leaving the door open."
As an actor, Singh's thirst for learning was legendary. For example, he studied the style and mannerisms of carriage riders to prepare for the role of a horse carriage driver in Inspector (1956).
Singh played prominent roles in movies like Jhoota Kahin Ka (1970), Haathi Mere Saathi (1971) and Mere Jeevan Saathi (1972). His last prominent role was in the 1973 film Loafer.
With advancing years, Singh became less active, particularly from the mid 1970s onwards. Many of his roles from the late 1970s onwards were mere cameo appearances, arranged with the sole purpose of ensuring that actors turned up on time- such was his stature that actors would never turn up late when K.N. Singh was in the sets. His last appearance was in Woh Din Aayega (1986).
Singh was the eldest among 6 siblings: a sister and five brothers. His marriage with prior spouses did not produce any children, and so the two adopted Pushkar, the son of his brother Bikram (who was once the editor of Filmfare magazine) as their son.
Singh became completely blind in his last years. He died in Mumbai on 31 January 2000 aged 91 was survived by his adopted son Pushkar, who is a producer of television serials.Baghban (1938)
Aap Ki Marzi (1939)
Humayun (1945) as Jai Singh
Funtoosh (1956) as Karodi Lal
Chalti Ka Naam Gaadi (1958)
Howrah Bridge (1958) as Boss
Singapore (1960) as Shivdas
Manzil (1960) as Mehta
Woh Kaun Thi? (1964) as Dr. Singh
Mere Huzoor (1968)
Jigri Dost (1969) as Chairman Neelkanth
Suhana Safar (1970) as Dr. Singh
Pagla Kahin Ka (1970) as Max
Himmat (1970) as Inspector Mathur
Ehsan (1970) as Dinanath/Vishwanath Prasad
Meeting (1970) as
Hum Tum Aur Woh (1971)
Dushmun (1971) as Public Prosecutor Saxena
Pyar Ki Kahani (1971) as Ram's boss
Haathi Mere Saathi (1971) as Sarwan Kumar
Jaane-Anjaane (1971) as Poonamchand
Mere Jeevan Saathi (1972) as Gobindram (Prakash's dad)
Do Chor (1972) as Tribhuvan Singh
Reshma and Shera (1972) as Chowdhary
Keemat (1973) as Chief of Secret Services
Hanste Zakhm (1973) as Chanda's patron
Daaman Aur Aag (1973) as
Loafer (1973) as Mr. Singh 'Uncle'
Kachhe Dhaage (1973) as Sona's father
Badhti Ka Naam Dadhi (1974) as Khadak Singh
Sagina (1974) as Factory Owner
Majboor (1974) as Prosecuting Attorney
Prem Kahani (1975) as Rai Bahadur Shrikant Sinha
Rafoo Chakkar (1975) as #3 Duo
Qaid (1975) as Dr. Trivedi
Kala Sona (1975) as Police Commissioner
Harfan Maulaa (1976) as Police Inspector
Aadalat (1976) as Singh
Saheb Bahadur (1977) as Police Supdt. Pasupathi
Jadu Tona (1977)
Agent Vinod (1977) as Chief of Agent Vinod
Guru Ho Jaa Shuru (1979) as CBI Chief (uncredited)
Do Premee (1980) as Shivlal/Mahayogi
Dostana (1980) as Judge
Professor Pyarelal (1981)
Shradhanjali (1981) as Amit's Diwanji
Kaalia (1981) as Convict
Teri Maang Sitaron Se Bhar Doon (1982) as Nataraj Seth
Woh Din Aayega (1987) as Police Commissioner hukumat 
Laat Saab (1992)