Although he was viewed by some as a religious radical during the Somerset Protectorate, he entertained Queen Elizabeth I of England at Cobham Hall in 1559, signalling his acceptance of the moderate regime.

At the end of the decade, he served in northern France, where his father was in charge of Calais, and in 1549, he accompanied William Paget's embassy to Brussels.

Like his father, Brooke sympathized with the anti-Marian nobles; he sided with the rebels during Wyatt's rebellion, and the intervention of his brother-in-law, Henry Nevill, 6th Baron Bergavenny, was needed to keep him from prison.

Brook became Warden of the Cinque Ports, a position in which he wielded great power over a large number of seats in Parliament.

Most important, the accession of Queen Elizabeth I, and his close friendship with William Cecil, 1st Baron Burghley, made him a powerful noble.

Along with William Cecil, he numbered among his friends some nobles, such as the Duke of Norfolk and the Earl of Arundel, whose loyalty to Queen Elizabeth was far from certain.

[2] During his time, William Brooke built Cobham Hall, a Tudor style mansion that remained in his family until the mid-17th century.

At an unknown date, Margaret went mad, and on 4 November 1602, it was reported that Doctor John Dee had been called in and 'hath delivered the Lady Sondes of a devil or of some other strange possession'.