Confidant of Edward III, key roles in start of Hundred Years War . First son of William de Montacute (?-1319) .

Received Lundy Isle, 1322; knighted and departed for France, 1325; with Edward III to repel Scottish invasion, 1327; ambassador to the Pope; close confident of the king, led group that used secret-passage in castle to find and violently arrest Mortimer in the queen-mother's apartment, 1330; Montacute pardoned for the death of two of Mortimer's attendants; with Edward III when, disguised as merchants they made a secret visit to France, 1331; with Edward III in Scotland at siege of Berwick and battle of Halidon Hill, 1333; received all Edward's right to Isle of Man, 1333; in co-command of the army in Scotland with Arundel, 1335; After unsuccessful siege of Dunbar Castle made a truce strongly disapproved of in England, 1336.

Made earl of Salisbury by Edward III, 1337; Made Admiral of the Fleet and sent to France to declare the English claim to the French throne and to raise an alliance against the French, 1337; raided Scotland, 1338; appointed marshal of England, 1338; in Flanders 1339-1340; negotiated with French at behest of Pope; taken prisoner attempting to take Lille castle surreptitiously with small force, 1340; life saved by King of Bohemia, ransomed on condition of never returning to France; conquered Isle of Man at some point in 1340-1342; to Spain (Castile) for siege of Algeciras fighting Moors with Alfonso XI.

Died from bruises during tournament at Windsor. Daughter Philippa married Roger Mortimer.

Details of how William was made earl of Salisbury are noted in the Amiens manuscript:

"The parliament also ordained and confirmed the marriage of Sir William Montagu, who had loyally served the King in the Scottish wars... To reward him for his services, the King gave him the young Countess of Salisbury, Madame Alys , whose estate he held in wardship. She was one of the most beautiful young ladies in the land." (Froissart Chronicles).

A wardship was the legal right, and responsibility, to decide who someone else will marry. This seems to have been a more complex matter then it at first appears. For more information, see the section on Katherine de Montacute.

William is referred to as both the first and sixth earl of Salisbury, because the Plantagenets had been the previous earls of Salisbury but had forfeited the title due to Thomas Plantagenet's conviction for treason in 1322.

Sources:
The DNB.
[DMKC].
Froissart Chronicles, trans. Geoffrey Brereton.
Lordship of England, Waugh.
[WUANK].

http://www.montaguemillennium.com/familyresearch/h_1344_william.htm