Lucy's siblings, Peter, Susan and Edmund, do not believe her about Narnia at first, but later they all find their way to Narnia. In the first story, The Lion, The Witch and The Wardrobe, she is the first of the Pevensies to enter Narnia through a magical wardrobe in Professor Kirke's old house, and sees Narnia in the One Hundred Year Winter, under the rule of Jadis the White Witch, the evil self-styled Queen of Narnia. There she meets Mr. Tumnus the Faun and, later, the Beavers. While traveling with her siblings and beavers, Father Christmas gives them gifts; Lucy is given a vial with magical cordial that can heal almost any injury , and a small dagger with which to defend herself "at great need". She and her companions arrive at Aslan's camp, and later that night, she and Susan comfort Aslan as he walks to his death. Both girls also witness his sacrifice.

While their brothers are going to war, Lucy and her sister see Aslan come back to life and help him wake the creatures in the White Witch's castle, which the White Witch had turned to stone. They meet with their brothers at the end of the battle. At Cair Paravel, she is crowned to the Glistening Easter Sea as Her Majesty Queen Lucy by Aslan to the throne as co-ruler of Narnia, thus marking the fulfilling of the ancient prophecy and the end of the White Witch's reign. During her reign, the people name her Queen Lucy the Valiant. She and her siblings make a Golden Age in Narnia. Late in the Golden Age while hunting the white stag through Lantern Waste, she notices the lantern where she met Mr. Tumnus and the children run through the wardrobe into England, where no time had passed and they are children again.

Lucy travels to Narnia again with her three siblings in Prince Caspian. In that book, Lucy is the only one to see Aslan at first, and she has a terrible time convincing her brothers and sister as well as Trumpkin the dwarf that he had returned, echoing her trials early in the first book (though Edmund ultimately trusts her, given that she had turned out to be right about Narnia during their last visit). Aslan tells her to try again, and says that she must follow him alone if they refuse to come with her. Lucy comments that Aslan has grown larger. He explains that he appears larger to her because she is older.

While Susan travels with Mr. and Mrs. Pevensie to America and Peter studies with Professor Digory Kirke, Lucy, Edmund and their cousin Eustace are drawn into Narnia through a magical painting in The Voyage of the Dawn Treader. This is very much Lucy's book, written largely from her point of view. However, at the end Aslan firmly tells her she has become, like Susan, too old to further experience the wonders of Narnia.