Seaxburh's sisters were Æthelburg of Faremoutiers, Saethryth, Æthelthryth and possibly Withburga. Her marriage to Eorcenberht produced two sons, both of whom ruled, and two daughters. After her husband's death in 664, Seaxburh remained in Kent to bring up her children. She acted as regent until her young son Ecgberht came of age.
Seaxburh founded the abbeys at Milton Regis and Minster-in-Sheppey, where her daughter Ermenilda was also a nun. She moved to the double monastery at Ely where her sister Æthelthryth was abbess and succeeded her when Æthelthryth died in 679. According to Bede, in 695, Seaxburh organised the movement (or translation) of Æthelthryth's remains to a marble sarcophagus, after they had lain for sixteen years in a common grave. On opening the grave, it was discovered that her body was miraculously preserved. The legend is described in Bede's Ecclesiastical History of the English People, which celebrates the saintly virtues of Æthelthryth, but speaks less highly of Seaxburh, referring only to her marriage, succession as abbess and translation of her sister's relics. The date of Seaxburh's death at Ely is not known. The surviving versions of the Vita Sexburge, compiled after 1106, describe her early life, marriage to Eorcenberht, retirement from secular life and her final years as a nun and abbess at Ely.
Seaxburh was a daughter of Anna, King of East Anglia, the son of Eni, who ruled the East Angles from the early 640s and was slain together with his son Jurmin, at the Battle of Bulcamp in 653 or 654. She was the sister of Æthelburg and Saethryth, who were both abbesses of Faremoutiers Abbey in Brie, and also the sister of Æthelthryth, who married firstly Tonberht, an ealderman of the South Gyrwe in the Fens, and secondly Ecgfrith of Northumbria. Withburga, who died in 743, may also have been her sister.
Seaxburh married Eorcenberht of Kent, and was the mother of kings Ecgberht (d. 673), Hlothhere (d. 685), and of Saints Eormenhild and Ercengota. Seaxburh was buried at Ely with her sisters Æthelthryth Wihtburh, and with her daughter Eormenhild.
Seaxburh was connected with the royal family of the Magonsætan by her marriage to Eorcenberht, who was king of Kent from 640 to 664. Eorcenberht was the great-uncle of Mildburh and her sisters, the daughters of King Merewalh of the Magonsætan.