Tumnus is the son of a faun who had fought in a failed revolution against the White Witch's Army, who like Tumnus himself, was born in the Age of Winter. He lives in a comfortable cave near the Lamp-post. During the last years of the reign of the White Witch. When Lucy Pevensie first enters Narnia through the Wardrobe, Tumnus is the first Narnian she meets; they bump into each other by accident at the Lamp-post. Tumnus had never seen a human, and after being informed by Lucy that she is in fact a Daughter of Eve (a human female), he invites her to his home for tea. He later reveals to Lucy, that he has an explicit knowledge of the previous era.
Along with many other Narnians, Tumnus is bent by fear to obey the laws of the White Witch Jadis, one of which is to inform her of or deliver to her, any Daughters of Eve or Sons of Adam that are found in Narnia. This is to prevent the fulfillment of the Golden Age Prophecy, which the Witch knows means her downfall. Although Tumnus resents the rule of the Witch, he is afraid to disobey her. He plans to lull Lucy to sleep by playing his pan pipes, kidnap her, and take her to the Witch.
Lucy and Tumnus have tea, and afterwards he plays her a tune on his Narnian pan flute, of which he is quite an accomplished player. She begins to nod off to sleep. However, he could not go through with the immoral plan. His talk with Lucy makes him realize what a heinous act it would be to betray her. Bursting into tears, he apologizes to Lucy, told her about the Witch, and risked his life to help her escape back through the Wardrobe to Earth.
Lucy visits Tumnus again on her second entrance to Narnia, when Edmund Pevensie also got in. However, the Witch's Secret Police found out about his helping Lucy escape, through their network of spies in the woods, and arrested him. He is taken to the Witch's castle for a supposed "trial". When the four Pevensie children arrive in Narnia together, they visit Tumnus' home and find it ransacked, but also that a foreboding note had been left:
"The former occupant of these premises, the Faun Tumnus, is under arrest and awaiting his trial on a charge of High Treason against her Imperial Majesty Jadis, Queen of Narnia, Chatelaine of Cair Paravel, Empress of the Lone Islands, etc., also of comforting her said Majesty's enemies, harboring spies and fraternizing with humans.'Signed MAUGRIM, Captain of the Secret Police, LONG LIVE THE QUEEN!"
It is unknown what Tumnus' trial consisted of, or if he ever truly received one. However it is known that Jadis turned him to stone.
Tumnus is rescued and "unpetrified" by the breath of Aslan when, after his resurrection, he liberates Witch's castle and "unpetrified" all the Narnians in it who had been turned to stone, by breathing on them.
Tumnus may have fought in the First Battle of Beruna, though he may also have supported the Narnian revolutionaries in other ways. He remains a close friend of the four Pevensie monarchs throughout the Golden Age.
The last reference to him is his reported sighting of the White Stag, which initiated the hunt for the stag that resulted in the four Pevensies' disappearances from Narnia (having returned to Earth).