Clive Staples Lewis was born in Belfast, Ireland (now Northern Ireland), to Albert James Lewis and Flora Augusta Hamilton Lewis. He had a brother, Warren Hamilton Lewis, three years his elder. He adopted the name "Jack" as a boy, simply because he liked the sound of it. From that point on, he was known by this nickname by close friends and family. Lewis' mother died in 1908, and he was sent to a number of different schools in England. In about 1913, he abandoned his childhood Christian faith. In 1929, he became a theist: "In the Trinity Term of 1929 I gave in, and admitted that God was God, and knelt and prayed..." Ultimately, in 1931, he returned to Christianity.

Lewis had a passion for "dressed animals" as a boy, falling in love with Beatrix Potter's stories and often writing and illustrating his own animal stories. He and his brother, Warnie, together created the world of Boxen, which was inhabited and run by animals. Lewis loved to read, and as his father's house was filled with books, he often felt that finding a book he hadn't read was as easy as finding a blade of grass. He also had a mortal fear of spiders and insects as a child, for they often haunted his dreams.

As a teenager, he was wonderstruck by Richard Wagner and the songs and legends of the North. They intensified a longing he had within him, a deep desire he would later call "joy". He also grew to love nature-- the beautiful scenes in nature reminded him of the stories of the north, and the stories of the north reminded him of the beauties of nature. In his teenage years, his writing moved away from the tales of Boxen, and he began to use new literary forms (poetry and opera) to try and capture his newfound interest in Norse myth and in the natural world.

In 1916 Lewis won a scholarship to University College, Oxford. However in 1917 he enlisted in the British Army, and was commissioned an officer in the 3rd Battalion, Somerset Light Infantry. He arrived at the front line in the Somme Valley in France on his 19th birthday. He was wounded during the Battle of Arras, and on his recovery assigned duty in England. He was discharged in December 1918, and returned to his studies. He received a First in Honour Moderations (Greek and Latin Literature) in 1920, a First in Greats (Philosophy and Ancient History) in 1922, and a First in English in 1923.

Lewis taught as a fellow of Magdalen College, Oxford for nearly thirty years, from 1925 to 1954, and later was the first Professor of Medieval and Renaissance Literature at the University of Cambridge.

The Chronicles of Narnia is a series of seven fantasy novels for children that is by far the most popular of his works. The books have Christian themes and describe the adventures of a group of children who visit a magical land called Narnia. The Lion, the Witch, and the Wardrobe, which was the first published and the most popular book of the series, has been adapted for both stage and screen. The Chronicles of Narnia borrow from Greek and Roman mythology, and traditional English and Irish fairy tales. Lewis reportedly based his depiction of Narnia in the novels on the geography and scenery of the Mourne Mountains in County Down, Northern Ireland. Lewis cited MacDonald as an influence in writing the series. The books were published in an order different from that they take place in. In chronological order, the seven books are: The Magician's Nephew, The Lion, the Witch and the Wardrobe, The Horse and His Boy, Prince Caspian, The Voyage of the Dawn Treader, The Silver Chair, and The Last Battle. Many people feel that the books should be read in order of publication, beginning with The Lion, the Witch and the Wardrobe (followed by Caspian, Dawn Treader, Silver Chair, Horse, Magician's Nephew and Last Battle, in that order).

Lewis died on November 22, 1963, at the Oxford home he shared with his brother, Warren. He is buried in the Headington Quarry Churchyard, Oxford, England.