English bookseller whose collection of printed books, handbills, pamphlets, ballads, newspapers, and other writings (cataloged and bound from 1640 to 1661) constitute one of the most important historical sources for the English Civil Wars and Commonwealth. The original collection reportedly contained more than 32,000 separate items; the extant “Thomason Collection” in the British Museum numbers 22,255 items in 2,008 leather volumes.

As a youth Thomason was apprenticed to a London bookseller (1617–26), after which he became a member of the Stationers' Company and set up shop in the area of St. Paul's Churchyard, where he lived and worked for the rest of his life, marrying (1631) and raising a large family. He was a close friend of the poet John Milton and the pamphleteer William Prynne and was a staunch Presbyterian (being briefly imprisoned in 1651 for his political activities).

Thomason started his collection at the outset of the struggles between the monarchy and Parliament and worked in secret because of the censorship existing throughout the subsequent changes of regime. The annotated collection was a personal legacy and brought him no income; and, though he was otherwise, as a publisher and bookseller, a major supplier of books to the University of Oxford and occasionally to Cambridge and was a frequent book-buying traveler to the European continent, he died in penury.

In 1761 Thomason's collection was bought from his descendants by George III, who presented it to the British Museum.