At first a courtier of James I of England, he would later strike up friendships with Charles I of England and his wife Henrietta Maria whom he hosted lavish banquets for. He was created Marquess of Newcastle-upon-Tyne and entrusted with the education of the royal couple's son, the future Charles II of England. Cavendish was a staunch royalist helping to fund the king in his Bishops' Wars and then during the English Civil War he was made a general for the fight in the North of England against the roundheads. After the defeat at Marston Moor, Cavendish went into self-imposed exile, only returning with the English Restoration where he was made a duke.
A descendent of the famous Cavendish family, William Cavendish was born at Handsworth Manor in Yorkshire to Sir Charles Cavendish and Catherine Ogle. On the paternal side of his family, the grandparents of William were Bess of Hardwick and courtier William Cavendish. Shortly after the birth of William, his brother Charles was born. It is not known exactly what did he was born, but the brothers would maintain a close relationship throughout their lives. The family lived at Welbeck Abbey.
On the occasion of the creation of Prince Henry as Prince of Wales in 1610, Cavendish was made a Knight of the Bath, subsequently travelled with Sir Henry Wotton, then ambassador to the Duke of Savoy, and on his return married his first wife, Elizabeth (before 1602-c. 1643), daughter of William Basset of Blore, Staffordshire, and widow of Henry Howard, third son of the 1st Earl of Suffolk. He possessed an immense fortune, and several times he entertained King James I and King Charles I with great magnificence at Welbeck and Bolsover.
Subsequently his plans were checked by Fairfax's re-capture of Leeds in January 1643, and he retired to York. He escorted the queen, who returned from abroad in February, to York, and subsequently captured Wakefield, Rotherham and Sheffield, though failing at Leeds, but his successes were once more ravished from him by Fairfax. In June he advanced again, defeated the Fairfaxes at Adwalton Moor on 30 June, and obtained possession of all Yorkshire except Hull and Wressel Castle.
The Duke might now have joined the king against Lord Essex, but continued his campaign in the north, advancing into Lincolnshire to attack the eastern association, and taking Gainsborough and Lincoln. Thence he returned to besiege Hull, and in his absence the force which he had left in Lincolnshire was defeated at Winceby by Oliver Cromwell on 11 October 1643, which caused the loss of the whole county. On 27 October 1643, he was created Marquess of Newcastle-upon-Tyne.
The next year Newcastle's position was further threatened by the advance of the Scots. Against larger numbers he could do little but harass and cut off supplies. He retreated to York, where the three armies of the Scots, as well as those of Lord Fairfax of Cameron and Lord Manchester, surrounded him. On 1 July 1644 Prince Rupert of the Rhine raised the siege, but on the next day threw away his success by engaging the three armies in battle, contrary to Newcastle's desire, at Marston Moor.
Newcastle left in 1648 for Rotterdam with the intention of joining the Prince of Wales in command of the rebellious navy, and finally took up his abode at Antwerp, where he remained till the Restoration. In April 1650 he was appointed a member of Charles II's privy council, and in opposition to Edward Hyde advocated the agreement with the Scots. In Antwerp he lived in the Rubenshuis (the house where the painter Peter Paul Rubens had lived from 1610 till his death in 1640) and established his famous riding-school, exercised "the art of manège" (High School riding), and published his first work on horsemanship, Méthode et invention nouvelle de dresser les chevaux (1658, 2nd edition, 1747; translated as A General System of Horsemanship, 1743). This work had an influence on one of the greatest French riding masters, Francois Robichon de la Gueriniere, as well as a more controversial figure in dressage, Baucher. He is also said to be the inventor of draw reins.
At the Restoration (1660) Newcastle returned to England, and succeeded in regaining the greater part of his estates, though burdened with debts, his wife estimating his total losses in the war at the enormous sum of £941,303. He was reinstated in the offices he had filled under Charles I; was invested in 1661 with the Order of the Garter which had been bestowed upon him in 1650, and was advanced to a dukedom on 16 March 1665. He retired, however, from public life and occupied himself with his estate and with his favourite pursuit of training horses. He established a racecourse near Welbeck. In his later years, he suffered from Parkinson's Disease, and the sudden death of his second wife was a blow from which he never recovered.
The Duke died at Welbeck Abbey on Christmas Day 1676, and was buried in Westminster Abbey. By his first wife he had ten children, of whom one son, Henry, survived him and became 2nd Duke of Newcastle-upon-Tyne, dying in 1691 without surviving male issue; the title then became extinct and the estates passed to his third daughter Margaret, wife of John Holles, Earl of Clare, created Duke of Newcastle-upon-Tyne in 1694. The 1st Duke's daughters included Lady Elizabeth Cavendish (c. 1627-14 June 1663), who married the 2nd Earl of Bridgwater and had issue, and Lady Frances Cavendish (before 1641-15 August 1678), who married the 2nd Earl of Bolingboke.
During their stay in Antwerp, the Cavendishes had a music chapel of 5 musicians. They were acquainted with several of the contemporary English composers, and Newcastle's library contained a substantial collection of music of these composers.
As a commander in the field Lord Clarendon spoke contemptuously of Newcastle as "a very lamentable man, and as fit to be a general as a bishop". It can hardly be denied, however, that his achievements in the north were of great military value to the king's cause. For politics he had no taste, and adhered to the king's cause merely from motives of personal loyalty, from hatred of "whatsoever was like to disturb the public peace," and because the monarchy "was the foundation and support of his own greatness". Even Clarendon conceded that he was "a very fine gentleman".
The department of Manuscripts and Special Collections, The University of Nottingham holds a number of papers relating to the 1st Duke: the Cavendish Papers (Pw 1), part of the Portland (Welbeck) Collection, includes some of his personal papers; the Portland Literary Collection (Pw V), also part of the Portland (Welbeck) Collection, contains many of his literary papers; and the Newcastle (Clumber) Collection (Ne) includes some estate papers from the time of the 1st Duke, for example, relating to his purchase of Nottingham Castle.