Margaret Leijonhuvfud was a member of one of Sweden's most powerful noble families: the early Leijonhufvud clan of Swedish nobility (the name meaning Lion's Head), being the daughter of Erik Abrahamsson Leijonhufvud, a man executed in the Stockholm bloodbath, and wife Ebba Eriksdotter Vasa, the second cousin of king Gustav.

She was expected to learn how to manage a large estate and landholding and perform the duties of her future spouse in his absence, as well as to have knowledge at least in the German language except Swedish, and to deport herself with humility but also dignity by reading religious literature.

Her sister Brita married the king's favorite courtier Gustav Olofsson Stenbock in the presence of the royal couple shortly after, and though no list of Queen Catherine's ladies survives, Margaret's social position, connections, age, and contemporary custom makes it extremely likely that she completed her education by serving the queen, as was the custom for girls of her position at the time.

[2] Margaret married King Gustav I October 1536 in Uppsala Cathedral in a ceremony conducted by archbishop Laurentius Petri and was crowned Queen there the next day.

[4] During the first years of their marriage, Margaret's mother Ebba played a dominating role in the royal court, and it was said that even the king did not dare to oppose his mother-in-law.

[2] Queen Margaret has been given a very good estimation in contemporary documents as well as in history, and referred to as intelligent and beautiful; she is described as a loyal wife who never abused her influence, as a responsible parent, a skillful manager of the royal court and household, and as a compassionate philanthropist of the poor and needing.

The members of the royal household are only fragmentarily known during her tenure, but she hosted a great number of maids-of-honour, who were successively married to the king's male courtiers in order to carefully balance the power among the noble families of the realm.

[2] Among the most prominent members of her own staff was the cunning woman Birgitta Lass Andersson, a trusted favorite and confidante with medical knowledge, who was entrusted many of her private affairs and also saw to the health of herself, her sister Martha and children.

[3] She is said to have been credited with a great diplomatic ability, which made it possible for her to have success rather than to irritate the king when she came to him and spoke to him on others' behalf in various issues, some of them indeed political.

Already in the autumn of 1536, a pardon for a criminal sentenced to execution was mitigated by the king "For the sake of the Prayer of Our Dear Mistress Queen Margaret", and she also managed to have the king return the confiscated property of Margareta Gire, wife of the exiled suspected conspirator Wulf Gyler, release her from custody and allow her to depart to her spouse in Germany.

[4] She was entrusted tasks within foreign policy: when her brother Sten was sent as ambassador to the French royal court in 1542 in order to negotiate a Swedish-French alliance, Margaret was tasked to perform a diplomatic correspondence with the French king's influential sister Marguerite de Navarre in Latin,[2] and she also attended the First Treaty of Brömsebro (1541).

[2] In his succession order issued in Västerås in 1544, he stated that if he should die when his successor was still a child, Margaret should rule as regent in a guardian government with representatives of the nobility until the age of majority of his son.

According to the chronicle of Aegidius Girs, Margaret thanked her spouse on her death bed for making her queen, regretted that she had not been worthy of it, and asked her children not to quarrel.