In May 1501, his father died and Sir Walter succeeded him as the 9th Baron Ferrers. On 7 December 1509 he was accorded special livery, although still under age, without proof of age or payment of relief for his father’s lands. He was appointed High Steward of Tamworth on 20 November 1510, and joint Constable of Warwick Castle on 15 Feb 1511 with Sir Edward Belknap. He was also Joint Steward of the Manor, or Borough of Warwick. On 27 January 1513, he was appointed Keeper of Netherwood Park; Councillor and Royal Commissioner of Wales and the Marches on 1 August 1513; and High Steward of Hereford in 1514. Walter Devereux served with distinction in the War of the League of Cambrai and the Italian War of 1521-1526. He was Captain of the English Army against France from 24 August 1523. For his gallantry in the sea battle off Le Conquet he was rewarded on 13 July 1523 by being created a Knight of the Garter by Henry VIII of England alongside diplomat Sir Thomas Boleyn, later Earl of Wiltshire. He was appointed Bailiff of Sutton Coldfield ; Steward of the Household and Counsellor to Mary, Princess of Wales ; Chief Justice of South Wales ; High Steward of Builth ; and Chamberlain of South Wales, Carmarthen and Cardigan. In the late 1520s, Devereux came into increasing conflict with Welsh magnate Rhys ap Gruffydd following King Henry's grants of major power to him in Wales as described above. This reached a head when Rhys, with a gang of armed supporters, threatened Ferrers with a knife in 1529. The two men were allowed to air their grievances, but Rhys's family continued to stir up trouble. Eventually Rhys was charged with treason, convicted and executed. The consolidation of Devereux's position in Wales helped prepare the way for the Reformation. Devereux served as Custos Rotulorum of Cardiganshire from 1543 to his death. He was with King Henry VIII when Boulogne was taken on 18 September 1544. Created Viscount Hereford on 2 February 1550, as well as sworn of the Privy Council, he was also appointed Vice-Lord Justice and Lieutenant of Stafford ; and Justice of the Peace for Stafford, Worcester and Salop. Lord Hereford died on 17 September 1558, and is buried at St. John's Church, Stowe, Staffordshire.
Marriages and children
Devereux was first married to Lady Mary Grey prior to his father's death in May 1501, and was pardoned on 15 December 1503 for having married in his father's lifetime and when under marriageable age. She was a daughter of Thomas Grey, 1st Marquess of Dorset and his second wife Cecily, suo jure Baroness Harington and Bonville, daughter and heiress of William Bonville, 6th Baron Harington by his wife Katherine Neville, suo jure Baroness Hastings. Her maternal grandparents were Alice Montagu, 5th Countess of Salisbury and Sir Richard Neville, jure uxoris Earl of Salisbury. Lord and Lady Ferrers had three children: