Her maternal grandparents were Charles Brandon, 1st Duke of Suffolk and Mary Tudor, former Queen consort of France. Mary was the daughter of Henry VII of England and Elizabeth of York and a younger sister of Henry VIII of England.
Mary Grey was described as the smallest person at court, crooked backed and 'very ugly'. Her reported deformity could be described as kyphosis.
Her oldest sister, Jane, was the designated heir of Edward VI of England, in a change to his will made on his deathbed. Edward VI died on July 6, 1553 and Jane was proclaimed Queen regnant on July 10. However Edward VI had by-passed his older half-sisters, Mary I of England, and Elizabeth I of England, an act which was of dubious legality, as it had not been approved by the Houses of Parliament. Jane was deposed in favor of Mary I on July 19, 1553, and executed on February 12, 1554. Mary I continued to reign until her natural death on November 17, 1558, and died childless. She was succeeded by her younger half-sister, Elizabeth.
Elizabeth was unmarried and childless. Lady Catherine Grey was considered a likely heir to the throne until her own death in 1568. This brought Lady Mary Grey to relative prominence. She was the last surviving grandchild of Mary Tudor, and considered by some to be heiress presumptive to the English throne. Mary Grey was already living under house arrest at that time, having been imprisoned in 1565 for marrying royal gatekeeper Thomas Keyes without the permission of Queen Elizabeth. She was released following his death in 1572 and was permitted to attend Court occasionally.
In spite of the intrigues involving her sisters, Mary Grey does not appear ever to have made a serious claim to the throne. She died childless at age 33.
The Sisters Who Would be Queen; the tragedy of Katherine, Mary & Lady Jane Grey, by Leanda de Lisle published January 2009, Harper Press UK