Eric XIV married Karin morganatically in 1567, and officially in 1568, when she was ennobled and crowned queen under the name Katarina Magnusdotter (a formal version of her name).
The first, secret wedding ceremony, was performed by Archbishop Laurentius Petri on 29 December 1567.
During this period, Eric was periodically affected by his mental problems, and a regency ruled in his stead.
The marriage, Eric's mental problems and Karin's elevation caused rumours.
In a letter to Eric's sister Sophia, it was reported that "The lunacy which dominates him is a consequence of his deeds.
He was somewhat unstable even before, and now he is completely so, guided by his wife", implying that Karin used witchcraft.
The wife of Eric's advisor Jöran Persson, Anna Andersdotter, a close friend of Karin, was accused of having spread the rumours and sentenced for slander, but was pardoned on request by Karin herself.
   
The regency arrested many of Eric's advisors, and Karin intervened for many of them.
When Eric recovered and the regency was disbanded, Karin intervened in making peace between Eric and his brother, Duke John.

The 10 February 1568, Karin was ennobled and given the official seal of a queen.
In a letter to her sisters-in-law, the princesses Sophia and Elisabeth, it is noted that she had erased the first draft, which formally called them "Dear sisters", and that she did not call herself "Queen", but "Chosen Queen", which has been interpreted to the fact that she knew that her position was not fully accepted within the aristocracy.
Eric is reported to have plans to have his brothers and other enemies killed before the wedding, but they were to have been warned of these plans by Karin Månsdotter through queen dowager Catherine Stenbock.
It was later noted that these people did not attend the wedding.

The official wedding took place in Storkyrkan on 4 July 1568, followed next day by the coronation.
Karin Månsdotter was walked to the altar by the king's cousin Per Brahe under a banner of golden textile carried by four nobles.
The wedding was unique; never before had the children of the couple been present at a royal wedding.
Both the infant son and the daughter of Eric and Karin were present to confirm their official status.
They were placed under the banner between their parents.
On 9 June the son had been baptized carried by Queen Dowager Catherine Stenbock.
The coronation was celebrated with great festivities in Stockholm to confirm the new queen's legitimacy.
 Karin's peasant relatives, her three maternal uncles from Uppland, Hans Jakobsson, Jakob Jakobsson and Erik Nilsson, were present dressed in clothes made for them by the royal tailor.
During the coronation, the Lord Chancellor Nils Gyllenstierna, who was carrying the crown, fainted and dropped the crown to the floor.
This was regarded as a bad omen.

During her period as queen, Karin made gifts to her friends and relatives, such as Anna Nilsdotter i Hammarstorp, who has been identified as her sister despite her different patronymicon, her paternal cousins in Sko kloster, and her "Fränka" (meaning "Female relative") Anna Eriksdotter i Sko.
Queen Karin is noted to have granted two royal estates in 1568: on 1 May, she granted the income from Rörstrand in Solna to Torbrjörn Klockare in Stockholm, and on 24 July, she granted Aleby in Botkyrka to Lasse Jakobsson, signing herself in the first document as "Karin Månsdotter" and in the second as "Katarina Regina".
On both occasions, she described different privileges and conditions for the holders.
Granting royal estates and deciding their privileges was something normally done by monarchs and described her personal influence.

Soon after the coronation, Eric's brothers rebelled and he was imprisoned.
Eric XIV married Karin morganatically in 1567, and officially in 1568, when she was ennobled and crowned queen under the name Katarina Magnusdotter (a formal version of her name). The first, secret wedding ceremony, was performed by Archbishop Laurentius Petri on 29 December 1567. During this period, Eric was periodically affected by his mental problems, and a regency ruled in his stead. The marriage, Eric's mental problems and Karin's elevation caused rumours. In a letter to Eric's sister Sophia, it was reported that "The lunacy which dominates him is a consequence of his deeds. He was somewhat unstable even before, and now he is completely so, guided by his wife", implying that Karin used witchcraft. The wife of Eric's advisor Jöran Persson, Anna Andersdotter, a close friend of Karin, was accused of having spread the rumours and sentenced for slander, but was pardoned on request by Karin herself.    
The regency arrested many of Eric's advisors, and Karin intervened for many of them. When Eric recovered and the regency was disbanded, Karin intervened in making peace between Eric and his brother, Duke John.
The 10 February 1568, Karin was ennobled and given the official seal of a queen. In a letter to her sisters-in-law, the princesses Sophia and Elisabeth, it is noted that she had erased the first draft, which formally called them "Dear sisters", and that she did not call herself "Queen", but "Chosen Queen", which has been interpreted to the fact that she knew that her position was not fully accepted within the aristocracy. Eric is reported to have plans to have his brothers and other enemies killed before the wedding, but they were to have been warned of these plans by Karin Månsdotter through queen dowager Catherine Stenbock. It was later noted that these people did not attend the wedding.
The official wedding took place in Storkyrkan on 4 July 1568, followed next day by the coronation. Karin Månsdotter was walked to the altar by the king's cousin Per Brahe under a banner of golden textile carried by four nobles. The wedding was unique; never before had the children of the couple been present at a royal wedding. Both the infant son and the daughter of Eric and Karin were present to confirm their official status. They were placed under the banner between their parents. On 9 June the son had been baptized carried by Queen Dowager Catherine Stenbock. The coronation was celebrated with great festivities in Stockholm to confirm the new queen's legitimacy.  Karin's peasant relatives, her three maternal uncles from Uppland, Hans Jakobsson, Jakob Jakobsson and Erik Nilsson, were present dressed in clothes made for them by the royal tailor. During the coronation, the Lord Chancellor Nils Gyllenstierna, who was carrying the crown, fainted and dropped the crown to the floor. This was regarded as a bad omen.
During her period as queen, Karin made gifts to her friends and relatives, such as Anna Nilsdotter i Hammarstorp, who has been identified as her sister despite her different patronymicon, her paternal cousins in Sko kloster, and her "Fränka" (meaning "Female relative") Anna Eriksdotter i Sko. Queen Karin is noted to have granted two royal estates in 1568: on 1 May, she granted the income from Rörstrand in Solna to Torbrjörn Klockare in Stockholm, and on 24 July, she granted Aleby in Botkyrka to Lasse Jakobsson, signing herself in the first document as "Karin Månsdotter" and in the second as "Katarina Regina". On both occasions, she described different privileges and conditions for the holders. Granting royal estates and deciding their privileges was something normally done by monarchs and described her personal influence.
Soon after the coronation, Eric's brothers rebelled and he was imprisoned.
Karin Månsdotter