Contemporaries does not describe her as intelligent, but as a moderate character with a good sense of tact and decorum and a cheerful temperament, lacking of any mind to plot and participate in intrigues at court.
She belonged to a family used to court service, her paternal aunt being the lady-in-waiting Margareta Brahe (1559–1638), and prior to her first marriage, she served as Hovfröken (maid-of-honour) to Queen Maria Eleonora.
[1] On 4 July 1633, she married the riksråd and royal equerry baron Bengt Oxenstierna (1591–1643) in Stralsund, where she was at the time still serving as maid-on-honour to Maria Eleonora, after a three years' engagement.
In 1648, the queen's cousin Countess Palatine Eleonora Catherine of Zweibrücken referred to her as her "Dearest Protection", likely because Margareta Brahe had defended her when she gave birth to an illegitimate child.
[4] Consequently, a party pamphlet accused Axel Oxenstierna for having arranged the marriage to create an alliance between himself and Margareta's brother Per Brahe and his fraction, which caused political conflicts.
"[4] But Johan Oxenstierna was genuinely in love, wished to marry Margareta exclusively for emotional reasons and refused to consider political, economic or fertility reasons, and in July 1648, Johan Oxenstierna and Margareta Brahe eloped to Wismar in Germany, where they were wed.[1] As the marriage was neither political, economic or intended to produce children, it was an obvious love match, which was controversial in an epoch where marriage was normally not conducted for love: it took place but four months after the death of the groom's first wife, and it also caused a scandal because of the age difference, when the bride rather than the groom was one decade senior.
The couple remained in Germany, where Johan Oxenstierna was a Swedish delegate during the Congress of Osnabrück, and Margareta Brahe reportedly played an important part during the Peace of Westphalia through her calming influence upon her hot-tempered husband, which benefited the sensitive negotiations.
[1] On 12 May 1661, she married Frederick II in a grand wedding with elaborate celebrations at the royal court in Stockholm in the presence of the king and the queen dowager regent.
[1] This conflict caused a scandal in all of Germany and "affected the emotions of Margareta Brahe to a degree, that she thought herself dying",[1] but through the mediation of her brother and several German Princes, the managed to effect a reconciliation with Louis Henry before his death in 1662.
"[4] This also caused a scandal, as she left almost nothing to her relatives, and the spouse of her niece, Adolph John I, Count Palatine of Kleeburg, opposed the will without success.