She is the 10-year-old Lady of Bear Island making her head of House Mormont of Bear Island. She is the niece of Lord Commander Jeor Mormont of the Night's Watch and cousin of Ser Jorah Mormont.
She is the daughter of Maege Mormont. She was named after Lyanna Stark, younger sister of Lord Eddard Stark, who died years before she was born. After her mother's older brother Jeor joined the Night's Watch and later his only son Jorah fled to Essos to avoid a death sentence by Lord Eddard Stark upon being caught selling poachers into slavery to fund his wife's expensive tastes, Maege Mormont became Lady of Bear Island.
Stannis Baratheon has come to Castle Black and send ravens to all the noble houses of the North with messages asking them to join his cause. Lyanna Mormont sends back a rebuff that she and Bear Island only bend the knee to House Stark. An upset Stannis shows Lyanna Mormont's message to Jon Snow which causes Jon to briefly smile and point out that, much like the Free Folk, the Northmen are loyal to their own.
Lyanna is the first Northern leader to be visited by Jon Snow, Sansa Stark, and Davos Seaworth, who are attempting to rally the North against House Bolton. Sansa and Jon try to flatter her, but Lyanna aggressively brushes them off, demands to know why they are at Bear Island, and notes that neither Jon nor Sansa are technically even Starks, the former being a bastard and the latter being twice married. She refuses to offer her men to them until Davos steps in. He tells her that the real war is not between different houses, but between the living and the dead - and a divided North cannot hope to stand against the Night King. She agrees to help, but can only offer sixty-two fighting men
A strong-willed girl of 10, Lyanna rebuffed Stannis Baratheon's call for troops from her homeland of Bear Island. Ser Davos Seaworth was able to convince the girl to support the Stark cause with the 62 fighting men of her house.