C.S. Lewis commonly used the name Jack after childhood.
Lewis was born Clive Staples Lewis, but he adopted the name Jack when he was about four years old. According to accounts of his childhood, the change followed the death of his dog Jacksie, whose name he began using for himself. The nickname eventually became shortened to Jack.
Family members, friends, and colleagues generally called him Jack rather than Clive or C.S. Lewis. His initials remained the public identity attached to his books, including The Chronicles of Narnia and Mere Christianity. This difference between his formal name and familiar name can surprise readers.
His older brother Warren was also known as Warnie. The brothers remained close, and Warren later became an important source for biographical information about Lewis. “Jack” is therefore not a pen name in the usual sense; it was the personal name by which people who knew him commonly addressed him.