Who was the last person executed in England under an Act of Attainder?

Sir John Fenwick, 3rd Baronet (c. 1645 – 28 January 1697) was an English Jacobite conspirator, who succeeded to the Baronetcy of Fenwick on the death of his father in 1676.

He began to plot against the new King William, for which he underwent a short imprisonment in 1689. Renewing his plots on his release, he publicly insulted Queen Mary in 1691, and it is practically certain that he was implicated in the schemes for assassinating William which came to light in 1695 and 1696. He arrest in June 1696.

To save himself he offered to reveal all he knew about the Jacobite conspiracies; but his confession was a farce, being confined to charges against some of the leading Whig noblemen, which were damaging, but not conclusive. By this time his friends had succeeded in removing one of the two witnesses, and in these circumstances it was thought that the charge of treason must fail. The government, however, overcame this difficulty by introducing a Bill of Attainder (also known as the Act of Attainder), which after a long and acrimonious discussion passed through both Houses of Parliament (Act 8 & 9 Will. III c. 4). His wife persevered in her attempts to save his life, but her efforts were fruitless, and Fenwick was beheaded in London on 28 January 1697, with the same formalities as were usually observed at the execution of a peer. He was the last person executed under an Act of Attainder.