English Roman Catholic imprisoned in the Tower of London from 1605 to 1621 on suspicion of complicity in the Gunpowder Plot (q.v.).

On the death of his father, the 8th earl, in 1585, he succeeded to the earldom and settled in London. Although an unavowed Catholic, he wished only open toleration for English Catholics and ardently favoured the accession of James I after the Scottish king had given assurances on the question.

During the first two years Northumberland supported the new regime but then fell out over the punishment of his friend Sir Walter Raleigh and the continued punishment of Catholics. How deeply or marginally he was involved in the Gunpowder Plot (in which his kinsman Thomas Percy played a leading role) is unclear, but he was arrested, tried in 1606, and sentenced to life imprisonment in the Tower. He was released on clemency in 1621 and took no further part in public affairs.