A Fellow of the Royal Society from 1665,[1] he was a Royalist supporter before being falsely implicated by Titus Oates in the later discredited "Popish Plot", and executed for treason.

[1] William was undoubtedly exposed to Roman Catholic influences, as almost all of the Howard family remained loyal in private to that faith, even if they conformed outwardly to the Established Church.

In 1620, William was placed in the household of Samuel Harsnett, Bishop of Norwich for an education, then attended St John's College, Cambridge, at age 11 in 1624, but did not receive a degree.

He was allowed by Parliament to return to England with his wife for a time in 1646 and 1647, but in 1649 his estates were sequestered and he was forced to compound for recusancy and royalism.

At his trial in 1680, he said vaguely that he might have promoted a policy of religious toleration in his speeches in the House of Lords, but could not remember this in any detail.

[1] His relative obscurity was held against him during the Plot; informers like Stephen Dugdale cunningly invented quite plausible speeches in which he lamented the King's ingratitude and the lack of reward the Howards had received for their loyalty.

Scepticism about the plot grew and it was thought that the imprisoned peers might be released, but anti-Catholic feelings revived in 1680 and Stafford was put on trial in November for treason.

There were several inconsistencies in his story, especially concerning the relevant dates, but Stafford, lacking expert legal assistance, failed to exploit them properly.

He failed, where a good defence counsel might have succeeded, in exposing the inconsistencies in the evidence of Turberville, or to discredit the unsavoury Oates, whose public standing had declined notably over the preceding year.

As Evelyn also noted Stafford was "not a man beloved by his own family", and seven out of eight peers of the Howard dynasty who sat on the Court voted him Guilty.

The King, even though he is not thought to have had much personal regard for the unpopular Stafford, later said that he had signed the death warrant "with tears in his eyes", but in the current state of public opinion, a reprieve was impossible.

[1] Gilbert Burnet wrote that he was quickly forgotten, but others thought that the publication of a version of his final words, addressed to his daughter Delphina (who was a nun at Leuven), in which he spoke eloquently of his innocence – "My good child, I pray God bless you.