Duchess and regent of Brittany
In 1092 or 1093, her father married her to Duke Alan IV of Brittany, probably to secure an alliance against Normandy, then controlled by William the Conqueror's son, Robert Curthose. Though her marriage was arranged for political reasons this does not mean that Ermengarde would have been seen as inferior to her husband. It was not uncommon that noble women in twelfth century France actually wielded more political power than their husbands. The union produced three children: the future Duke Conan III, Hawise and, Geoffrey. It is also suggested that during her time as duchess, Ermengarde may have raised her half-brother Fulk V of Anjou alongside her own children.
Ermengarde, seemingly unhappy in her marriage to Alan IV, repeatedly attempted to have her marriage annulled so that she could focus on religious rather than public life. Her requests were denied by religious authorities on the grounds that she did not have a witness to support her claim of consanguinity. Robert of Arbrissel, whom she kept correspondence with, told Ermengarde that it was her "duty to remain in the world, despite your desire to leave it, and to fulfill God's will concerning you." This letter from Robert of Arbrissel includes over 100 biblical references as well as references to religious leaders and Roman poets, showing that Robert knew Ermengarde to be intelligent and well read. It was not uncommon that noblewomen were better educated and more literate than noblemen during the Middle Ages as women were often expected to maintain their own properties and those of their husbands.
Ermengarde was also known to receive advice from other religious leaders of the time including Geoffrey of Vendome and St. Bernard of Clairvaux. Some scholars such as Shawn Madison Krahmer have noted that the letters surviving from St. Bernard of Clairvaux to Ermengarde show a level of intimacy and fondness that would suggest a close friendship between the two rather than a simply advisory correspondence.
When her husband left in 1096 to take part in the First Crusade, Ermengarde served as Regent of the Duchy of Brittany from 1096 until 1101. Despite being a time of political instability in Brittany, Ermengarde proved to be a popular and effective ruler maintaining stability during the Duke's absence by utilizing her familial ties and alliances. Ermengarde remained active in ruling alongside Alan upon his return from the crusades. Ermengarde was also effective in her political role by having close ties and a level of influence over the church, becoming an important supporter of reforms. In a letter to Ermengarde Geoffrey of Vendome praised her abilities as a leader saying that she was "fighting for God rather than caught up in worldly affairs."
She spent little time in Rennes or the west of Brittany, preferring Nantes and the Saumur region. Influenced by Robert of Arbrissel, she approved the expansion of the abbey at Fontevraud, to which she withdrew from 1103 to 1105 due to marital issues with Alan. In 1112, Ermengarde's husband, Alan, abdicated his position as Duke and entered the monastery of Redon as a monk where he remained until his death. Ermengarde continued to rule the county alongside her son, Conan III for many years.