Thomas Percy was a member of the group of provincial English Catholics who planned the failed Gunpowder Plot of 1605. A tall, physically impressive man, little is known of his early life beyond his matriculation in 1579 at the University of Cambridge, and his marriage in 1591 to Martha Wright. In 1596 his second cousin once removed, Henry Percy, 9th Earl of Northumberland, appointed him constable of Alnwick Castle and made him responsible for the Percy family's northern estates. He served the earl in the Low Countries in about 1600–1601, and in the years before 1603 was his intermediary in a series of confidential communications with King James VI of Scotland.

The Gunpowder Plot of 1605, in earlier centuries often called the Gunpowder Treason Plot or the Jesuit Treason, was an unsuccessful attempted regicide against King James I by a group of English Catholics led by Robert Catesby who considered their actions attempted tyrannicide and who sought regime change in England after decades of religious persecution.