Blathwayt was baptized in the parish of St Martin-in-the-Fields in London on 2 March 1649, the only son of William Blathwayt, barrister, of the Middle Temple, and his wife Anne Povey, daughter of Justinian Povey of Hounslow, Middlesex, who was accountant-general to Queen Anne of Denmark.

[2] Blathwayt joined the diplomatic service in 1668 when his uncle Thomas Povey, an influential London lawyer, found him a post as Clerk of the English embassy at The Hague.

Also in 1679, he was considered "as a very fit person" to be assistant to the secretary of the council, being heavily involved in the administration of England's colonies in North America.

He did much to promote trade in America and benefited considerably from gifts and bribes received in connection with his office (as was the usual practice in his day).

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.

[2] Blathwayt was a witness for the prosecution at the Trial of the Seven Bishops in 1688 and he lost the politically sensitive post as secretary at war after the Glorious Revolution.

[4] Blathwayt 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.

[4] His descendants sold a large part of his art collection in 1765, but some of the paintings have been purchased back or remain at Dyrham Park.