He was born in Faridpur in what is now Bangladesh. He studied at the Surendranath College, Dumdum Motizhil College , City College, all affiliated with the University of Calcutta. Thereafter, he obtained his Master's degree in Bengali from the University of Calcutta in 1954.

He was the founder editor of Krittibas, a seminal poetry magazine started publishing from 1953, that became a platform for a new generation of poets experimenting with many new forms in poetic themes, rhythms, and words. Later, he started writing for various publications of the Ananda Bazar group, a major publishing house in Kolkata and has been continuing it for many years. He became friends with the beat poet Allen Ginsberg while he was traveling in India. Ginsberg mentioned Gangopadhyay most notedly in his poem September on Jessore Road. Gangopadhyay in return mentioned Ginsberg in some of his prose work. After serving five years as the Vice President, he was elected the President of the Sahitya Akademi on February 20, 2008

Sunil, along with Tarun Sanyal, Jyotirmoy Datta and Satrajit Dutta had volunteered to be defense witnesses in the famous trial of ???? ? ??????? (Hungryalism) poet Malay Roy Choudhury.

He married Swati Bandopadhyay on February 26, 1967. Their only son, Sauvik was born on November 20, 1967.

Author of well over 200 books, Sunil is a prolific writer who has excelled in different genres but declares poetry to be his "first love". His Nikhilesh and Neera series of poems (some of which have been translated as For You, Neera and Murmur in the Woods) have been extremely popular.

As in poetry, Sunil is known for his unique style in prose. Arjun, Pratidwandi, filmed by Satyajit Ray (English title: The Adversary), Aranyer Din-Raatri (The Days and Nights of the Forest, also filmed by Satyajit Ray), Ekaa ebong Koyekjon are some of his well known works of fiction. His historical fiction Sei Somoy (translated into English by Aruna Chakravorty as Those Days) received the Indian Sahitya Akademi award in 1985. Sei Somoy continues to be a best seller more than two decade after its first publication. The same is true for Pratham Alo (also translated recently by Aruna Chakravorty as First Light), another best selling historical fiction and Purbo-Paschim, a raw depiction of the partition and its aftermath seen through the eyes of three generations of Bengalis in West Bengal, Bangladesh and elsewhere. He is also the winner of the Bankim Puraskar (1982), and the Ananda Puraskar (twice, in 1972 and 1989).

Sunil has written (and still writes) in many other genres including travelogues, children's fiction, short stories, features, and essays. Among his pen-names are: Nil Lohit, Sanatan Pathak, and Nil Upadhyay.

Though he has written all types of children's fiction, one character created by him that stands out above the rest, is Kakababu, the crippled adventurist, accompanied by his young adult nephew Shantu, and his friend Jojo. Since 1974, Sunil Gangopadhyay has written over 35 novels of this popular series, most of which appeared in Anandamela magazine.

Novels

His first novel was Atmaprakash. His other novels include:

• Aranyer Dinratri
• Sei Samay
• Rokto
• Arjun
• Purush
• Agniputro
• Sorol Satya
• Byaktigoto
• Pratidwandi
• Mohaprithibi
• Roktomangsho
• Bandhubandhab - an unusual work on friendship
• Purba-Paschim (in two parts) - an epic novel on east and west Bengal
• Jeeban Je Rakam
• Ardhek Manobi
• Pratham Alo
• Nihsanga Samrat

Kakababu series

• Sabuj Dwiper Raja
• Kakababu O Sindukrahasya
• Kakababu O Bajralama
• Santu Kothay,Kakababu Kothay
• Vijaynagarer Hire
• Jangaler Modhe Ek Hotel
• Bhayankar Sundoor

Poetry

• Hathat Nirar Janya