In November 1660 by his father's death he had become Viscount Valentia and Baron Mountnorris in the Irish peerage, and on the 20th April 1661 he was created Baron Annesley of Newport Pagnell in Buckinghamshire and earl of Anglesey in the peerage of Great Britain.
In the bitter religious controversies of the time Anglesey showed great moderation and toleration.
In 1681 Anglesey wrote A Letter from a Person of Honour in the Country, as a rejoinder to the earl of Castlehaven, who had published memoirs on the Irish rebellion defending the action of the Irish and the Roman Catholics.
In so doing Anglesey was held by Ormonde to have censured his conduct and that of Charles I.
In 1682 he wrote The Account of Arthur, Earl of Anglesey.