William Blathwayt (or Blathwayte) (1649 – 1717) was a civil servant and politician who established the War Office as a department of the British Government and played an important part in administering the English (later British) colonies of North America.
Born in the parish of St Martin-in-the-Fields in London, Blathwayt was born to a well-to-do family of Protestant merchants and lawyers. His father, William Blathwayt senior, was educated at Brasenose College, Oxford and attended the Middle Temple. William junior followed the same route, enrolling at the Middle Temple in 1665. He joined the diplomatic service in 1668 when his uncle Thomas Povey, an influential London lawyer, found him a post at the English embassy in The Hague.
Returning to London in the early 1670s, Blathwayt became an Clerk of the Privy Council in Extraordinary. He was considered "as a very fit person" to be assistant to the secretary of the council, becoming heavily involved in the administration of England's colonies in North America. In 1680, he became the first auditor-general of royal revenues in America, and after 1685 became the secretary of the Privy Council's committee on trade and foreign plantations—in effect, colonial under-secretary. It was in this capacity that he became a key figure in American affairs. He was responsible for establishing the charter of the Crown colony of the Province of Massachusetts Bay, the predecessor of the state of Massachusetts. He did much to promote trade in America and the Caribbean, promoting the slave trade and benefiting considerably from gifts and bribes received in connection with his office (as was the usual practice in his day).
In October 1686, he became a Clerk of the Privy Council in Ordinary. That same year, he married Mary Wynter, a wealthy heiress. His rise was noted by many of his contemporaries; the diarist John Evelyn commended him as "very dexterous in business" and as one who had "raised himself by his industry from very moderate circumstances."
In 1683, Blathwayt obtained by purchase the office of Secretary at War. This was originally merely the role of secretary to the Commander-in-Chief of the British Army but under Blathwayt the remit of the Secretary was greatly expanded to encompass all areas of Army administration. He effectively established the War Office as a department of the government, although he had very little input into the actual conduct of wars. Issues of strategic policy during wartime were managed by the Northern and Southern Departments (the predecessors of today's Foreign Office and Home Office respectively). He was a witness for the prosecution at the Trial of the Seven Bishops in 1688.
He became a Whig Member of Parliament for Bath in 1693 (a post which he retained until 1710).
He built a large mansion house for himself at Dyrham Park near Bristol, which he decorated with numerous Dutch Old Masters and sumptuous fabrics and furnishings. His descendents sold a large part of his art collection in 1765, but some of the paintings have been purchased back or remain at Dyrham Park.
Blathwayt retired to Dyrham in 1710 (his wife had died in 1691). He remained there until his death in 1717; he is buried in the local churchyard.