Lagertha's career as a warrior began when Frø, king of Sweden, invaded Norway and killed the Norwegian king Siward. Frø put the women of the dead king's family into a brothel for public humiliation. Hearing of this, Ragnar Lodbrok came with an army to avenge his grandfather Siward. Many of the women Frø had ordered abused dressed themselves in men's clothing and fought on Ragnar's side. Chief among them, and key to Ragnar's victory, was Lagertha. Saxo recounts:

"Ladgerda, a skilled Amazon, who, though a maiden, had the courage of a man, and fought in front among the bravest with her hair loose over her shoulders. All-marveled at her matchless deeds, for her locks flying down her back betrayed that she was a woman." x