C.S. Lewis used the nickname Jack from childhood onward.
The name arose after the death of his childhood dog, Jacksie. Lewis reportedly disliked the sound of his own name being used for the dog and began calling himself Jacksie instead. The nickname was later shortened to Jack, and it remained his preferred personal name for the rest of his life.
This explains why biographies and memoirs often refer to “Jack” rather than “C.S. Lewis.” “C.S.” was his public literary name, while Jack was the name used by friends, family, and colleagues. The distinction can confuse readers encountering personal letters or reminiscences for the first time.
His brother Warren, usually called Warnie, also used Jack. The nickname therefore belongs to Lewis’s private identity rather than being a pen name. His published books generally appeared under C.S. Lewis, preserving the initials that became internationally recognizable.