Mary remained in England for most of her childhood, until she was sent abroad in 1514 around the age of fifteen when her father secured her a place as maid-of-honour to the King's sister, Princess Mary, who was going to Paris to marry King Louis XII of France.
After a few weeks, many of the Queen's English maids were sent away, but Mary was allowed to stay, probably due to the fact that her father was the new English ambassador to France. Even when Queen Mary left France after she was widowed on 1 January 1515, Mary remained behind at the court of Louis's son-in-law and daughter, Francis I and Claude.
She returned to England in 1519, where she was appointed a maid-of-honour to Catherine Of Aragon, the queen consort of Henry VIII.
Soon after her return, Mary was married to William Carey, a wealthy and influential courtier, on 4 February 1520; Henry VIII was a guest at the couple's wedding. At some point, Mary became Henry's mistress; the starting date and duration of the liaison are unknown.
Anne had returned to England in January 1522; she soon joined the royal court as one of Queen Catherine's maids-of-honour. Anne achieved considerable popularity at court, although the sisters already moved in different circles and were not thought to have been particularly close.
Although Mary was said to have been more attractive than her sister, Anne seems to have been more ambitious and intelligent. When the king took an interest in Anne, she refused to become his mistress, being shrewd enough not to give in to his sexual advances and returning his gifts. By the middle of 1527, Henry was determined to marry her. This gave him further incentive to seek the annulment of his marriage to Catherine Of Aragon. A year later, when Mary's husband died during an outbreak of sweating sickness, Henry granted Anne Boleyn the wardship of her nephew, Henry Carey. Mary's husband had left her with considerable debts, and Anne arranged for her nephew to be educated at a respectable Cistercian monastery. Anne also interceded to secure her widowed sister an annual pension of £100.
In 1532, when Anne accompanied Henry to the English Pale of Calais on his way to a state visit to France, Mary was one of her companions. Anne was crowned queen on 1 June 1533 and on 7 September gave birth to Henry's daughter Elizabeth, who later became Queen Elizabeth I. In 1534, Mary secretly married an Essex landowner's younger son: William Stafford (later Sir william Stafford). Since Stafford was a soldier, his prospects as a second son so slight, and his income so small, many believed the union was a love match. When Mary became pregnant, the marriage was discovered. Queen Anne was furious, and the Boleyn family disowned Mary. The couple were banished from court.
Mary's life between 1534 and her sister's execution on 19 May 1536 is difficult to trace. There is no record of her visiting her parents, and no evidence of any correspondence with, or visits to, her sister Anne or her brother George when they were imprisoned in the Tower of London. Like their uncle, Thomas Howard, 3rd Duke of Norfolk, she may have thought it wise to avoid association with her now-disgraced relatives.
Mary died of unknown causes, on 19 July 1543, in her early forties.
Mary's marriage to William Stafford (d. 5 May 1556) may have resulted in the birth of two further children:
Mary was played by Valerie Gearonwas in the 1969 film Anne of the Thousand Days, and by Perdita Weeks in the Showtime television series The Tudors (2007–2010).
The Other Boleyn Girl was made into a BBC television drama in January 2003, starring Natascha McElhone as Mary and Jodhi May as Anne.