He lost his seat in parliament in 1702 owing to the unpopularity caused by the trial of his brother Spencer Cowper on a charge of murder.

[1] On 11 October 1705 he was sworn to the Privy Council,[2] was appointed Lord Keeper of the Great Seal, and took his seat on the woolsack without a peerage.

In November of that year he succeeded to his father's baronetcy, and on 14 December 1706, he was raised to the peerage as Baron Cowper of Wingham, Kent.

[1] He was the youngest Lord Keeper for many years: the Queen, who had taken a great liking to him, joked that "she had given the Seals to a boy" and suggested that in future he should wear a wig to lend him gravity.

[3] When the union with Scotland came into operation in May 1707 the Queen in Council named Cowper Lord High Chancellor of Great Britain, he being the first to hold this office.

[1] It is notable that Queen Anne, despite her prejudice against the Whigs in general, came to have great respect and liking for Cowper, and continued to seek his advice even after he left office as Lord Chancellor.

Spencer was one of the managers of the impeachment of Henry Sacheverell, was Attorney-General to the Prince of Wales (1714), Chief Justice of Chester (1717), and Judge of the Common Pleas (1727).

[6] The 1st Earl left two sons and two daughters by his second wife: Elizabeth Culling was the mistress of William Cowper, and bore him two children.