Son of William II, Prince of Orange, and Mary Stuart (daughter of Charles I). Born in The Hague. Married another grandchild of Charles I, Mary II (daughter of James II). William was formally invited by seven senior political men (the Immortal Seven), representing English Protestants, to invade Britain in order to replace the unpopular Catholic King James II. This Glorious Revolution went extremely well: William and his army landed on 5 November 1688 in Devon, James quickly fled to France and William and Mary were crowned joint monarchs on 11 April 1689. They converted the mansion of Nottingham House into Kensington Palace to serve as their home. By luck he was William III of both Orange and of England, though only the second king William of Scotland.
It's well-known that William died at Kensington Palace after being thrown by his horse tripping over a mole-hill. His delighted enemies, Jacobite adherents, subsequently toasted "the little gentleman in black velvet". Actually, all he suffered from that fall on 21 February was a broken collar-bone. He was recovering from that when a pulmonary fever got him.
Like the kings before him, he was governor of and invested in a company operating in the Atlantic slave trade.