In 1297 her grandfather Ralph de Monthermer had married Joan of Acre, to the displeasure of her father King Edward I.
He had three children with Joan, and after her death, he was appointed Baron Monthermer.
One of his sons was Thomas de Monthermer, Margaret's father.
[2] Thomas de Monthermer died in 1340 whilst fighting in the Battle of Sluys.
[1][3] Upon the death of her father, Margaret became suo jure Baroness Monthermer at the age of 10.