Mary Seymour (30 August 1548 - c. 1550) was the only daughter of Thomas Seymour, Baron Seymour of Sudeley, and Catherine Parr, widow of Henry VIII of England. Although Catherine was married four times, Mary was her only child. Complications from Mary's birth would claim the life of her mother on 5 September 1548, and her father was executed less than a year later for treason against Edward VI.
In 1549, the Parliament of England passed an Act (3 & 4 Edw. 6 C A P. XIV) removing the attainder placed on her father from Mary, but his lands remained property of the Crown.
As her mother's wealth was left entirely to her father and later confiscated by the Crown, Mary was left a destitute orphan in the care of Katherine Willoughby Duchess of Suffolk, who appears to have resented this imposition. After 1550 Mary disappears from historical record completely, leading to the conclusion that she did not live past the age of two.
Read more about Mary Seymour: Speculations of An Adult Life, Fiction, Ancestry