Never a queen herself, Mary was nevertheless both daughter of a king (Charles I of England and Ireland) and mother of a king (William III of England and Ireland, and II of Scotland).
While Mary was still quite young, she was highly sought after on the royal marriage market. In 1641, at the age of ten, she married Willem, son and heir to the Prince of Orange. However, Mary's husband then returned to Europe, and it is thought that the marriage was not consummated for some time after that.
In 1642, Mary's father Charles I gave her the title of "Princess Royal," a title that is now traditionally bestowed on the eldest daughter of a monarch, especially when there are male heirs (Mary had two brothers). Charles created the title to imitate the French tradition of calling the eldest daughter of a king "Madame Royale." That same year, Mary and her mother Queen Henrietta Maria crossed over to the Dutch republic, where Mary joined her father-in-law's household and began her active public life.
In 1647 Mary's husband Willem succeeded his father to his various titles (including Prince of Orange). In 1650, just after trying to take over Amsterdam, Willem died of smallpox; their only child (Willem, later William III) was born a few days later. Without her husband's protection, Mary became quite unpopular, owing primarily to her Stuart loyalties and the welcome she gave to her brothers in exile, Charles II and the Duke of York (later James II). Mary was forced for some time to share the guardianship of her son with his grandmother; but in 1657 she became regent, on his behalf, of the principality of Orange. Mary then sought assistance from Louis XIV of France to deal with her difficult Dutch relations; ironically, however, Louis seized the opportunity to grab control of Orange.
Mary's brother (Charles II) was ultimately restored as King, and she and Willem returned to England in 1660; unfortunately, however, she died of smallpox the same year. Politics being what they were in those days, Willem ultimately ascended to the British throne (as William III) in 1689, but only after his uncle James II was deposed.
William III reigned jointly with his second wife, Mary II (pictured above). They are the "William and Mary" after whom counties and colleges have been named in this country ....
.... including the College of William and Mary, which they chartered in 1693 as the second college in the American colonies.