Since lions are supposed to be "the kings of beasts", the Cowardly Lion believes that his fear makes him inadequate. He does not understand that courage means acting in the face of fear, which he does frequently. Only during the aftereffects of the Wizard's gift, when he is under the influence of an unknown substance, is he not filled with fear. He argues that the courage from the Wizard is only temporary, although he continues to do brave deeds while openly and embarrassedly fearful.
The Cowardly Lion makes his first appearance in the book The Wonderful Wizard of Oz. He is the last of the companions Dorothy befriends on her way to the Emerald City. The Cowardly Lion joins her so that he can ask The Wizard for courage, being ashamed that, in his cultural role as the King of the Beasts, he is not indeed brave. Despite outward evidence that he is unreasonably fearful, The Cowardly Lion displays great bravery along the way. During the journey, he leaps across a chasm on the road of yellow brick multiple times, each time with a companion on his back, and the leap back to get the next one. When they come into another, wider chasm, the Cowardly Lion holds off two Kalidahs while the Tin Woodman cuts a tall tree to cross it. In spite of his fears, he still goes off to hunt for his food, and he even offers to kill a deer for Dorothy to eat, but the idea makes her uncomfortable.
The Wizard gives him a bowl of liquid from a bottle marked "Courage," and in the remainder of the book, becomes almost like a bully and ready to fight. He accompanies Dorothy on her journey to see Glinda, and allows his friends to stand on his back in order to visit the Dainty China Country, where he damages the only church mentioned in an Oz book until Handy Mandy in Oz (1937).
His favored companion is the Hungry Tiger. This may well be the "Biggest of the Tigers" he and his friends encounter in the Forest of Wild Beasts in the Quadling Country. In this forest, all of the lions and many of the other animals have been eaten by a giant Spider. The Lion finds the Spider asleep and decapitates it. The Tiger and the other animals bow to him and ask him to be their king, and he promises to do so upon his return from accompanying Dorothy to Glinda. Glinda orders the Winged Monkeys to carry him back to the Forest once Dorothy has returned home.
In the rest of Baum's Oz series, the Lion was never again cast in a major role, used only as a minor character. In later books, The Cowardly Lion often accompanies Dorothy on her adventures. He is Princess Ozma's chief guardian on state occasions. In subsequent Oz books by Baum, the Lion was shown to have continued being courageous and loyal, although still considering himself a coward and regularly frightened, even by Aunt Em. He befriended the Hungry Tiger in Ozma of Oz, if this was not the earlier Tiger (which The Patchwork Girl of Oz implies that it is by calling both Lion and Tiger "largest of their kind"), and the two have become Ozma's personal guards. In Glinda of Oz he is on Ozma's board of advisers.
In "The Cowardly Lion and the Hungry Tiger" in Little Wizard Stories of Oz, the Lion begins with cowardly bravado, intending to find a man to tear apart, and the Tiger a fat baby to devour. Instead, they find a small child (bigger than a baby) and return it to is mother.
In his titular novel by Ruth Plumly Thompson, Mustafa of Mudge, a wealthy sultan at the southern tip of the Munchkin Country, kidnaps the Cowardly Lion for his large collection of lions that he feels would be incomplete without Oz's most famous lion. He was turned to stone by the giant, Crunch, but rescued by American circus clown Notta Bit More and orphan Bobbie Downs, whom the clown prefers to call by the more optimistic-sounding Bob Up.
For the most part in later Oz books, though, the Cowardly Lion is a presence rather than a major character. His other significant appearances include Ojo in Oz, where he is turned into a clock by Mooj and saved by Ozma and the Wizard. He assisted against the Stratovanians in Ozoplaning with the Wizard of Oz, Terp the Terrible in The Hidden Valley of Oz, and accompanied Dorothy and Prince Gules of Halidom in Merry Go Round in Oz. John R. Neill played him primarily as a beast of burden in his three Oz books. In all, the only three books in which the Cowardly Lion does not rate at least a mention are The Tin Woodman of Oz, Grampa in Oz, and The Silver Princess in Oz, though his absence in the second Oz book, The Marvelous Land of Oz, beyond a reminiscence, Baum reported in his introduction to Ozma of Oz to have brought letters of encouragement to bring him back.
A skittish and fearful lion cub is seen at Shiz University in Gregory Maguire's novel, Wicked: The Life and Times of the Wicked Witch of the West. The cub had been the result of cruel experiments by Dr. Dillamond's replacement teacher (in the musical, it was an agent of the Wizard) and was saved by Elphaba and some other students. This is heavily hinted to be a younger form of the Cowardly Lion. The Tin Woodman confirms this in the Broadway musical adaptation Wicked, in the song "March of the Witch Hunters": "And the lion also has a grievance to repay! If she'd let him fight his own battles when he was young, he wouldn't be a coward today!"
The book Lion of Oz and the Badge of Courage and its accompanying animated feature, Lion of Oz, show the Lion as having grown up in a circus in America. His caretaker, Oscar Diggs, was the man who would become the Wizard of Oz; this man took the Lion on a balloon ride one night, which resulted in the two becoming stranded in Oz.