This Earl, the eldest son and heir of William, Lord Montacute, the 1st Earl of Salisbury of that family, by Katherine daughter of William, Lord Grandisson, was born on the 20th June 1328. His father, one of the most eminent warriors of his time, died on the 30th January 1344, in consequence of bruises received at the Windsor jousts; and the young Earl, then in his sixteenth year, having doubtless also participated, and with distinction, in those chivalrous exercises, was chosen to be one of the Founders of the Order of the Garter in the April following.

In 1340, when only twelve, he was contracted in marriage to the Lady Joan Plantagenet, 'the Fair Maid of Kent'. Later, he was part of the expedition into France in 1346, receiving a knighthood, if not the degree of banneret, upon landing at La Hogue. We find him mentioned as assisting at the Siege of Caen and it may be presumed that he was also at Crécy. Upon his return, his steward, Sir Thomas Holland, petitioned Pope Clement VI, alleging that she had previously been the Earl's wife, in virtue of a marriage lawfully solemnised, and that, during his absence in distant parts, the Earl had married and then unjustly detained her from him. The case was referred, by the Holy See, to the investigation of Cardinal Ademar, while William continued to serve the King in France. Having examined witnesses on both sides over a period of two years, the Cardinal reported that the marriage between Holland and Joan had been legally celebrated. The Pope thereupon, by his bull dated at Avignon, 13th November 1349, decreed the contract with Montacute to be null and void, and ordered restitution of the lady to Holland, her lawful husband.

In the same year, William made proof of his age and had livery of his lands, and, before the end of the year, succeeded, upon the death of his mother, to the lands which she had held in dower. In 1350, he was in the naval engagement with the Spaniards off Winchelsea and, on 24th October 1353, did homage to the King at Westminster, in the presence of Prince Edward, for his Barony of Denbigh. Being, along with his younger brother, Sir John Montacute, of that Prince's retinue, he embarked at Plymouth for Gascony, on 1st January 1356 in order to play his part in the Hundred Years War against France. He had, with him, letters to the Seneschal for his special protection against any demand upon him during two years, on account of the debts of his ancestors. William was involved in the foray, with the Earls of Warwick, Suffolk and Oxford, in Languedoc, on which occasion they burnt the suburbs of Narbonne, destroyed Carcassonne and returned, over the district of Armagnac, to Bordeaux.

In 1356, the Earl commanded the rear of the English army at the Battle of Poitiers and continued in France during the year following. In 1359, he appears to have been in attendance on the King in his French expedition; and was there also in 1360. He was present, in 1368, at the conclusion of the truce and, in 1369, was sent, with the Earl of Warwick and others, under the command of the Duke of Lancaster, to Calais. In 1370, he was, at Westminster, one of the witnesses to the celebrated letter for the redress of grievances in Aquitaine. In 1372, he embarked with the King at Southampton and sailed towards Rochelle with a view to relieve Thouars; but the fleet was compelled, by contrary winds, to return to England. In 1376, he was constituted Admiral of the Fleet and, in the same year, was found by inquisition to be one of the co-heirs of his cousin, Thomas, Lord Grandisson.

On the accession of Richard II, William was appointed to secure the sea-coasts in the counties of Hampshire, Dorset & Somerset and, in the following year, was Governor of Calais. In 1384, he was ordered to march against the Scots. In 1385, the Isle of Wight and the Castle of Carisbrooke were granted to him for life. He continued in public employment until his death, which happened on 3rd June 1397, at the age of sixty-nine. He made his will on 20th April 1387, under the titles of Earl of Salisbury and Lord of the Isles of Man and Wight, and directed his interment in the conventual church of the Priory of Bisham in Berkshire, which had been founded by his father adjoining the family home. The will was proved on 27th June following.

The Earl had, soon after the decision of the Pope in 1349, married Elizabeth, the eldest daughter and co-heiress of John, Lord Mohun of Dunster (Somerset), by whom he had an only child, Sir William Montacute, who married, in 1378, Elizabeth FitzAlan, daughter of Richard, Earl of Arundel, but died without issue, having been unhappily slain in a tilting match at Windsor (Berks) by the Earl, his father, on the 6th August 1382. William was therefore succeeded in the Earldom by his nephew, John. Elizabeth, Countess of Salisbury, took the veil some years after the death of her husband; and was received into the sisterhood of the convent of St. Albans on 10th October 1408. She made her will on the eve of St. Katherine 1414, and died on 14th January 1415, leaving Philippa, Duchess of York, her younger sister, and Richard, Lord Strange of Knockyn, son of Matilda, her other sister, as her next heirs.

Edited from George Frederick Beltz's 'Memorials of the Most Noble Order of the Garter' (1841)