Raymond Scott, 55, was serving an 8-year term at Northumberland Prison for possessing the rare volume of plays.
The prison service said Scott, who passed himself off as a wealthy playboy before his conviction, was pronounced dead after being found unconscious in his cell Wednesday, and that the cause of death will be investigated.
Scott was a cigar-loving, self-described alcoholic who was jailed in 2010 after he took the 1623 volume of plays to the Folger Shakespeare Library in Washington, D.C., and asked to have it authenticated.
Experts there alerted police, who said the folio had been stolen from a display case at Durham University in northern England in 1998.
Scott, who arrived for various court appearances in a horse-drawn carriage led by a piper and later in a silver limousine, claimed he had found the volume in Cuba. He denied all charges against him.
A jury cleared Scott of stealing the First Folio but found him guilty of handling stolen goods and removing stolen property from Britain.
Prosecutors said the collector drove a yellow Ferrari and posed as an international playboy, despite living with his elderly mother on welfare benefits and amassing huge credit-card debts.
Scholars consider the Shakespeare folio one of the most important printed works in the English language. The First Folio was published seven years after Shakespeare's death and was the first collected edition of his plays. Fewer than 250 copies of the collection survive and most are incomplete.