William Montacute's date of birth is unknown, and is, not terribly helpfully, estimated at anywhere between 1265 and 1285. I'd put it roughly in the middle of that range, maybe sometime between 1275 and 1280. (The ODNB puts it at c. 1285, but that strikes me as too late given that William was almost certainly a father by 1300 and that he was old enough to act as his father's attorney in October 1302.)  William married Elizabeth, daughter of Sir Peter or Piers de Montfort of Beaudesert, Worcestershire (died 1287), and Maud de la Mare; Elizabeth's grandfather, also Peter de Montfort, was killed fighting with Simon de Montfort, earl of Leicester, at the battle of Evesham in 1265, though apparently the families were not related or only very distantly. William himself was the son of Sir Simon Montacute, a landowner in Somerset first summoned to parliament in 1299, almost certainly by Simon's first wife Hawise St Amand, and had to wait a long time to receive his inheritance: Simon lived until September 1317. The Montacute family came to England with William the Conqueror and were settled in Somerset by 1086.
William Montacute and Elizabeth de Montfort had four sons:
1) John Montacute, the eldest, born at the end of the 1200s or beginning of the 1300s. In Edward II's presence at Windsor on 28 April 1317, the same day Margaret Gaveston married Hugh Audley, John married Joan de Verdon (born August 1303), niece of Roger Mortimer and stepdaughter of Edward II's niece Elizabeth de Burgh. Sadly, John died shortly afterwards, and was buried in Lincoln Cathedral on 14 August 1317. Edward II paid forty clerks to pray for his soul and thirteen widows to watch over his body. The king arrived at Lincoln three days after the funeral, and gave generous alms at the masses celebrated in the cathedral for the repose of John’s soul.  Edward seems to have been particularly affected by John's death; perhaps this is a sign of his great affection for John's father, or grief at the sudden death of a promising young man so recently married, or both.
2) William Montacute, his father's heir after John's death and born between 1301 and 1303; he was said to be seventeen or eighteen in May 1320, and was granted part of his inheritance in May 1321 although still under-age and the rest on 21 February 1323.  William Jr was a close personal friend of Edward III (ten or so years his junior), was instrumental in the arrest of Roger Mortimer in October 1330, was rewarded with the earldom of Salisbury in 1337, and died in January 1344. William married Katherine Grandisson in about 1327 and had half a dozen children, including his heir, yet another William (1328-1397); Elizabeth, wife of Hugh Despenser the Younger's son and heir Hugh; Philippa, countess of March, wife of Roger Mortimer's namesake grandson and heir; John, who married Thomas de Monthermer's daughter and heiress Margaret; and Sybil, who married Edmund Fitzalan, disinherited grandson and namesake of the earl of Arundel executed in November 1326.
3) Simon Montacute, born in 1303 or 1304 (he was said to be in his fifteenth year and a student at Oxford in letters sent by Edward II to Pope John XXII on 28 November 1318 ), bishop of Worcester 1333, bishop of Ely 1337, died June 1345.
4) Edward Montacute, died July 1361, who was probably named in honour of Edward I or II and who married Alice of Norfolk, younger daughter and co-heiress of Edward II's half-brother Thomas of Brotherton, earl of Norfolk. In about 1351, Edward beat up his wife so badly she died of her injuries. His punishment for this atrocious act? None whatsoever.
William Montacute and Elizabeth de Montfort also had at least seven daughters: Maud (died 1352), one of the many royal and noble women of the Middle Ages to be elected abbess of Barking; Isabel (died 1358), who succeeded her sister as abbess of Barking (their niece, Edward Montacute's daughter Maud, became abbess in 1377); Elizabeth, prioress of Haliwell; Hawise, who married Sir (Roger?) Bavent; Katherine, who married Sir William Carrington; Alice, who married Ralph, Lord Daubeney; Mary, who married Sir Richard Cogan. Elizabeth de Montfort outlived her husband by many years, and died in 1354. On 8 June 1322, two years and eight months after William's death, she was pardoned, in exchange for a payment of £200, for marrying Sir Thomas Furnival without royal licence.  Thomas's first wife was Hugh Despenser the Elder's sister Joan, and in 1318 his son and heir Thomas married Joan de Verdon, teenaged widow of Elizabeth de Montfort's son John Montacute. William Montacute, Elizabeth and their son William, earl of Salisbury are the main characters in Juliet Dymoke's confusingly-named (but pretty good) 1979 novel The Lion of Mortimer.
William was named as a 'king's yeoman' in 1302 and 1303 and took part in Edward I's Scottish campaigns of the early 1300s - in March 1303, he was appointed to "supervise the shipping for the Scots war" - and was one of the men knighted with the future Edward II at Westminster on 22 May 1306.  In the early years of Edward II's reign, William's father Simon was more prominent than he was, and Edward appointed Simon constable of Beaumaris Castle on Anglesey on 1 March 1309. On 18 December 1311, William Montacute was one of the knights - with, among others, Edward II's brother-in-law Ralph de Monthermer, William le Latimer, John Cromwell and Hugh Audley Sr - who acted as mainpernors for his father, then imprisoned at Windsor Castle for "diverse felonies and trespasses." This is probably explained by an entry on the Patent Roll of 2 April 1313, which pardons Simon, on account of good service to Edward I and II, "for his attempt to occupy the land of Man to the king's disinheritance."  Three months previously, William Montacute had been appointed constable of Berkhamsted Castle, then held by Edward II's stepmother Queen Marguerite; the king was then switching the custody of key castles to men he trusted, two days after he was forced to consent to Piers Gaveston's third exile. 
William, by then a knight of the royal household, accompanied Edward II and Queen Isabella on their trip to France in the summer of 1313 and seems to have been on pretty good terms with the king around this time, to judge by the number of requests and petitions granted 'on the information of William de Monte Acuto'.  He was keeper of the vital port of Berwick-on-Tweed in the eighth year of Edward II's reign, 1314/15, but it was after 1315 that his closeness to and influence over Edward really becomes apparent. William was an excellent soldier: according to the Vita Edwardi Secundi, he commanded the royal cavalry, and on or shortly after 12 November 1315, Edward II sent him to liberate Maud, Lady Clifford, taken prisoner at Barnard Castle by John the Irishman. William had returned to court at Clipstone by 6 December, "having made the rescue aforesaid." (Maud Clifford, romantically, subsequently married Sir Robert de Welle, another of her rescuers.)  In February 1316, taking 150 men-at-arms and 2000 footmen with him, William was one of the knights Edward sent on the campaign against Llywelyn Bren, and the king received a letter from William on or shortly before 11 March informing him of the campaign's progress. William told Edward that "your bailiff of Gloucester has served you falsely, for where he should have brought 100 footmen he only brought us 48...". He described the men the bailiff had brought as "worthless rascals" (raskaille de nyent) and, a man of prompt action looking out for the king's best interests, told Edward that he had handed the bailiff over to the sheriff, "till your will be known." He informed Edward shortly before 24 March that he and the rest of the royal army "came to the Black Mountain, which is a little from the castle of Kayrfily [Caerphilly]...and we went to the end of the mountain a good three leagues from the roads and took the mountain and passed along it among all their force and Lewelyn and a great part of his host took to flight and those who stayed were soon dead and discomfited, and then we went to the castle and garnished it sufficiently with people and victuals and took the lady* out of the castle with us...". (Rather too many 'ands' and not enough commas in that sentence, William.) After Llywelyn's surrender, William was one of the three men appointed to "take fines and ransoms from all those who rose in insurrection with Llewellyn Bren," and the only man ordered to "receive into the king's peace" all those who had risen. 
[* 'the lady' = Maud de Clare née de Burgh, daughter of the earl of Ulster, sister-in-law of Robert Bruce and widow of Edward II's nephew the earl of Gloucester, who had the misfortune to be in Caerphilly Castle when Llywelyn attacked it; she was then pretending to be pregnant by her late husband, to the great annoyance of Hugh Despenser the Younger.]
William was rewarded for his loyalty to Edward II later in 1316, and became a dominant political figure at the king's court and an enemy of the earl of Lancaster...lots more about this coming soon, in the second part of the post!
1) Calendar of Patent Rolls 1301-1307, p. 67.
2) Thomas Stapleton, 'A Brief Summary of the Wardrobe Accounts of the tenth, eleventh, and fourteenth years of King Edward the Second', Archaeologia, 26 (1836), p. 339.
3) Oxford Dictionary of National Biography.
4) Foedera 1307-1327, pp. 379-380.
5) Calendar of Fine Rolls 1319-1327, p. 133.
6) Cal Pat Rolls 1301-1307, p. 128; Calendar of Close Rolls 1301-1307, p. 92; Calendar of Chancery Warrants 1244-1326, pp. 175, 206, 213; Constance Bullock-Davies, Menestrellorum Multitudo: Minstrels at a Royal Feast, p. 186.
7) Cal Pat Rolls 1307-1313, p. 102 (Beaumaris); Cal Close Rolls 1307-1313, p. 389 (mainpernors); Cal Pat Rolls 1307-1313, p. 565 (pardon).
8) Cal Fine Rolls 1307-1319, p. 103.
9) Cal Pat Rolls 1307-1313, pp. 502-504, 572, 575, 579.
10) Vita Edwardi Secundi, ed. N. Denholm-Young, p. 68; Cal Close Rolls 1323-1327, pp. 440-441.
11) Cal Chancery Warrants 1244-1326, pp. 437, 439-440; Cal Pat Rolls 1313-1317, pp. 384, 433, 444; Cal Fine Rolls 1307-1319, p. 274.