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.