Santosh's debut movie was 'Ithu Njangalude Katha' in 1982. Later he became one of the prominent faces of the Malayalam movie industry. Most of his roles were having a negative touch. Yuvajanolsavam, Peruvannapurathe Visheshangal, Kauravar, Sainyam, etc are some of his notable movies. He is married to Subashree.

Santhosh hails from Trivandrum. He is the only son of retired headmaster C. N. Kesavan Nair and P. Rajalakshmiamma, who is also a retired teacher. He has two sisters. He grew up along with his sisters in Pettah, Thiruvananthapuram. Santhosh was brought up by his maternal grandparents Adv. K. S. Chellappan Pillai and B. Parukuttyamma ever since his parents left to Ethiopia on deputation. He did his schooling at St. Joseph's Higher Secondary School, Thiruvananthapuram. Both his sisters turned out to be gynecologists while he wanted to pursue a career as an actor. He graduated in Maths from Mahatma Gandhi College, Thiruvananthapuram. He was active in politics during his college days. He got married to Subhasree.V, a high school teacher by profession and have a daughter, Rajasree S Nayar.