The eldest son of Henry, 2nd Earl of Sussex, by his first wife, Elizabeth, daughter of Thomas Howard, 2nd Duke of Norfolk, he was born about 1525, and after his father's succession to the earldom in 1542 was styled Viscount Fitzwalter. After serving in the army abroad, he was employed in 1551 to negotiate a marriage between King Edward VI of England and a daughter of Henry II of France. Radclyffe's prominence in the kingdom was shown by his inclusion among the signatories to the letters patent of June 16, 1553 settling the crown on Lady Jane Grey as Edward's successor; but he nevertheless won favour with Queen Mary, who employed him in arranging her marriage with Philip II of Spain, and who created him Baron Fitzwalter in August 1553.
Returning to England from a mission to Charles V, Holy Roman Emperor in April 1556, Fitzwalter was appointed lord deputy of Ireland. The prevailing anarchy in Ireland, a country which, nominally subject to the English Crown, was torn by feuds among its practically independent native chieftains, made the task of the lord deputy a difficult one; the difficulty was increased by the ignorance of English statesmen concerning Ireland and Irish conditions, and by their incapacity to devise any consistent and thoroughgoing policy for bringing the island under an orderly system of administration.
The measures enjoined upon Fitzwalter by the government in London comprised the reversal of the partial attempts that had been made during the short reign of Edward VI to promote Protestantism in Ireland, and the "plantation" by English settlers of that part of the country then known as Offaly and Leix. But before Fitzwalter could attend to such matters he had to make an expedition into Ulster, which was being kept in a constant state of disturbance by the Highland Scots from Kintyre and the Islands who were making settlements along the Antrim coast in the district known as the Glynnes (glens), and by the efforts of Shane O'Neill dominate more territory.
Having defeated O'Neill and his allies, the MacDonnells, the lord deputy, who by the death of his father in February 1557 became Earl of Sussex, returned to Dublin, where he summoned a parliament in June of that year. Statutes were passed declaring the legitimacy of Mary I of England, reviving the laws for the suppression of heresy, forbidding the immigration of Scots, and vesting in the Crown the territory comprised in what were later known as the King's County and Queen's County, which were then so named after Philip and Mary respectively. Having carried this legislation, Sussex endeavoured to give forcible effect to it, first by taking the field against Donough O'Conor, whom he failed to capture, and afterwards against Shane O'Neill, whose lands in Tyrone he ravaged, restoring to their nominal rights the Earl of Tyrone and his reputed son Matthew O'Neill, Baron of Dungannon. In June of the following year Sussex turned his attention to the west, where the head of the O'Briens had ousted his nephew Conor O'Brien, Earl of Thomond, from his possessions, and refused to pay allegiance to the Crown; he forced Limerick to open its gates to him, restored Thomond, and proclaimed The O'Brien a traitor. In the autumn of 1558 the continued inroads of the Scottish islanders in the Antrim glens called for drastic treatment by the lord deputy. Sussex laid waste Kintyre and some of the southern Hebridean isles, and landing at Carrickfergus he fired and plundered the settlements of the Scots on the Antrim coast before returning to Dublin for Christmas.
In the metropolis the news reached him of the queen's death. Crossing to England, he took part in the ceremonial of Queen Elizabeth's coronation in January 1559; and in the following July he returned to Ireland with a fresh commission, now as lord lieutenant, from the new queen, whose policy required him to come to terms if possible with the troublesome leaders of the O'Neills and the MacDonnells. Shane O'Neill refused to meet Sussex without security for his safety, and having established his power in Ulster he demanded terms of peace which Elizabeth was unwilling to grant. Sussex failed in his efforts to bring Shane to submission, either by open warfare or by a shameful attempt to procure the Irish chieftain's assassination.
He was preparing for a fresh attempt when he was superseded by the Earl of Kildare, who was commissioned by Elizabeth to open negotiations with O'Neill, the result of which was that the latter repaired to London and made formal submission to the queen. Shane's conduct on his return to Ireland was no less rebellious than before, and energetic measures against him became more imperative than ever. Having obtained Elizabeth's sanction, Sussex conducted a campaign in the summer of 1563 with Armagh as his temporary headquarters; but except for some indecisive skirmishing and the seizure of many of O'Neill's cattle, the operations led to no result and left Shane O'Neill with his power little diminished. His continued failure to effect a purpose for the accomplishment of which he possessed inadequate resources led Sussex to pray for his recall from Ireland; and his wish was granted in May 1564. His government of Ireland had not however, been wholly without fruit.
Sussex was the first representative of the English Crown who enforced authority to any considerable extent beyond the limits of the Pale; the policy of planting English settlers in Offaly and Leix was carried out by him in 1562 with a certain measure of success; and although he fell far short of establishing English rule throughout any large part of Ireland, he made its influence felt in remote parts of the island, such as Thomond and the Glynnes of Antrim, where the independence of the native septs had hitherto been subjected not even to nominal interference. His letters from Ireland display a just conception of the problems with which he was confronted, and of the methods by which their solution should be undertaken; and his failure was due, not to lack of statesmanship or of executive capacity on his own part, but to the insufficiency of the resources placed at his command and want of insight and persistence on the part of Elizabeth and her ministers.
On his return to England, Sussex, who before leaving Ireland had to endure the indignity of an inquiry into his administration instigated by his enemies, threw himself into opposition to Robert Dudley, Earl of Leicester, especially in regard to the suggested marriage between Leicester and the queen. He does not appear to have incurred Elizabeth's displeasure, for in 1566 and the following year she employed him in negotiations for bringing about a different matrimonial alliance which he warmly supported, the proposal that she should bestow her hand on the archduke Charles. When this project failed, Sussex returned from Vienna to London in March 1568, and in July he was appointed Lord President of the North, a position which threw on him the responsibility of dealing with the rebellion of the Earls of Northumberland and Westmorland in the following year. The weakness of the force at his disposal rendered necessary at the outset a caution which engendered some suspicion of his loyalty; and this suspicion was increased by the counsel of moderation which he urged upon the queen; but in 1570 he laid waste the border, invaded Scotland, and raided the country round Dumfries, reducing the rebel leaders to complete submission. In July 1572 Sussex became lord chamberlain, and he was henceforth in frequent attendance on Queen Elizabeth, both in her progresses through the country and at court, until his death.
The earl of Sussex was one of the great nobles of the Elizabethan period. Though his loyalty was questioned by his enemies, it was as unwavering as his patriotism. He shone as a courtier; he excelled in diplomacy; he was a man of cultivation and even of scholarship, a patron of literature and of the drama on the eve of its blossoming into the glory it became soon after his death. He was twice married: first to Elizabeth, daughter of Thomas Wriothesley, Earl of Southampton; and secondly to Frances, daughter of Sir William Sidney. His second wife was the foundress of Sidney Sussex College, Cambridge, which she endowed by her will, and whose name commemorates the father and the husband of the countess. The earl left no children, and at his death his titles passed to his brother Henry.
See PF Tytler, England under the Reigns of Edward VI and Mary (2 vols., London, 1839); Richard Bagwell, Ireland under the Tudors (3 vols., London, 1885-1890); Calendar of the Carew Manuscripts; John Stow, Annales (London, 1631); Charles Henry Cooper, Athenae cantabrigienses, vol. i. (Cambridge, 1858), containing a biography of the earl of Sussex; John Strype, Ecclesiastical Memorials (Oxford, 1822); Sir Cuthbert Sharpe, Memorials of the Rebellion of 1569 (London, 1,840); John Nichols, Progresses and Public Processions of Queen Elizabeth (3 vols., London, 1823); Sir William Dugdale, The Baronage of England (London, 1675).