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
Lady Lyanna is present at a meeting of Jon Snow's allied forces. When Lord Robett Glover protests at the King in the North's orders that everyone over the age of 10 be trained for combat, regardless of gender, Lyanna counters that she will train all on Bear Island against the coming White Walkers, famously stating that she does not intend to sit by her fire knitting while men fight to defend her. Lord Glover, not to be outdone by an 11-year old, withdraws his complaint.
When Jon is called to Dragonstone by Daenerys Targaryen, regarding an alliance, Lyanna states that with winter here, the North needs their King with them. As recompense, Jon leaves Sansa in control of the North, as he departs with Davos.
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.
Despite her young age, Lyanna is shown to be a competent, effective, and intelligent leader, in sharp contrast to the psychopathic Joffrey, timid Tommen, and developmentally challenged Robin Arryn. While she emanates self-confidence, she does not hesitate to consult her advisors if she sees the need. At the same time, she is not overly-reliant on her advisors, and will wave aside her maester if she has reached a firm decision already. As a female and the only living heir of a major Northern house, Lyanna challenges and strives to live up to the expectations placed upon her, and behaves in a stern, no-nonsense manner when negotiating with other houses. She is extremely proud of her house, and she is devoted to protecting Bear Island's people, indicating that she views this responsibility with the utmost solemnity. Though Lyanna shares her family's fierce loyalty to House Stark, she was initially reluctant to help Jon and Sansa fight the Boltons until Ser Davos convinced her, comparing their recent rises to power and informing her of the greater threat beyond the Wall.