She was the wife of Llywelyn the Great (initially king of Gwynedd), effective ruler of all of Wales.

[8] The infant was captured by English forces the following year, after her father, Llywelyn ap Gruffydd, was killed in December 1282.

When writing to the pope, attempting to secure more money for Sempringham Priory, the king stated that "...herein is kept the Princess of Wales, whom we have to maintain".

While her husband was briefly given various titles, including prince of Wales, by an Act of Parliament as part of his arrangement to succeed Henry VI, he is not generally recognised as such and is not mentioned in any published summary of the topic.

For most of her childhood, Mary was her father's only legitimate child, and for this reason, she was often referred to as the Princess of Wales, although Henry never formally created her as such.

For example, contemporary scholar Juan Luis Vives dedicated his Satellitium Animi to "Dominæ Mariæ Cambriæ Principi, Henrici Octavi Angliæ Regis Filiæ" ("Lady Mary, Prince of Wales, Daughter of Henry VIII, King of England").