John fitz Gilbert was the father of William Marshal Earl of Pembroke. John was the son of Gilbert, the marshal of the royal household of King Henry I. The office of the marshal was part of the Curia, with a deputy in the Exchequer and one in the King's Bench, as well as one in the Court of the Marshalsea of the King's household. The office was subordinate to the office of constable of the royal household. The office was responsible for everything connected to the horses of the royal household, the hawks and the hounds as well. He had the general duty of keeping order in the royal court/household, arranging for the billeting of members of the court, keeping tallies and other vouchers of the expenditures of the household, keeping rolls of all who performed their military service there, and being responsible for the imprisonment of debtors. The "Constitutio Domus Regis" gives the duties of the master marshal for King Henry I.

Both John and his father are found in the king's court before 1130 where they maintained [probably by trial by battle] their office of master of the king's marshalsea against William de Hastings and Robert de Venoiz. On the pipe roll of 1130 John is found paying twenty-two pounds for seisin to his father's lands and ministerium and forty marks for the office of marshal of the court. In this same year John married the daughter and heiress of Walter Pipard, a minor Wiltshire landholder. John was a loyal and trusted royal official and attested to at least twelve royal acts of Henry I between 1129-1135, most of them in England but some in Normandy.

When Stephen took the English throne on the death of Henry I in 1135, John continued to serve in the office of marshal and accompanied Stephen to Normandy in 1137. In 1138 John took possession of the castles of Marlborough and Ludgershall in Wiltshire as castellan and proceeded to strengthen both. During the early years of the war between King Stephen and the Empress Mathilda, John was more or less content to wait and watch, increasing the number of knights bound to him and fortifying his castles. He used his position in Wiltshire to attack and ravage the lands of those opposing King Stephen, though according to some of the chronicles of the times, John was not too particular about whom he attacked.

In February 1141, King Stephen was captured at the battle of Lincoln by Robert of Gloucester, natural brother to the Empress. This event apparently convinced John that he should be on the Empress's side in the civil war, and he actively supported her from this time forward. John was with the Empress at Reading in May, Oxford in July, and at the siege of Winchester in August 1141. When Henry of Blois, bishop of Winchester and brother to King Stephen, brought troops to relieve the siege of Winchester, it was decided that the Empress would flee to John's castle of Ludgershall with John while Robert of Gloucester continued the battle. At the village of Wherwell, John sent the Empress on to his castle with Brian fitz Count, and he stayed with some men to defend her retreat at the River Test. At the end of this struggle at the river, only John and one of his knights were left standing. They retreated to the church of Wherwell Abbey, and the enemy set fire to it. The enemy departed from Wherwell thinking that John had perished, but he survived and made it to his castle of Marlborough, losing one eye from melting iron in the fire.

The rising and falling fortunes of neither side in this civil war greatly effected fitz Gilbert's prosperity; he used his position and his castles in Wiltshire to continue to attack the lands of Stephen's supporters. One of his frequent victims was Patrick constable of Salisbury, who was King Stephen's man. After several years of this warfare, both men had had enough of the deprivations resulting from their attacks on each other. They worked out a compromise in 1141; John fitz Gilbert would put aside his first wife and marry Patrick's sister Sibile [Sibyl], and Patrick came over to the Empress's side. This compromise gave Patrick peace and relief as well as the later title and lands of the earldom of Salisbury. John nullified his most dangerous enemy and definitely increased his own social position by marrying into one of the great feudal families of England. It hurt neither man that they could both now raid the lands of Stephen's supporters in Wiltshire, Berkshire and Hampshire.

John was in high favor with the Empress, and she appointed his brother William as her chancellor. John himself witnessed at least four charters of the Empress, and there are two writs addressed to John in Wiltshire by her. He also witnessed five charters of Duke Henry in Normandy. October 25, 1154, King Stephen died and on December 19, 1154, Henry was crowned King Henry II of England. Henry II gave to John the manors of Marlborough, Wexcombe, and Cherhill in Wiltshire; they yielded eight-two pounds annually in revenues. He retained the office of marshal of the royal household. Along with these lands and the lands of his father, John held seven other knights' fees: land of the bishop of Winchester, of the bishop of Exeter, of the bishop of Winchester, of the abbot of Abingdon, of Richard de Candos[Chandos], of Manasser de Arsic, and of Geoffrey de Mandeville. He held Tidworth in Wiltshire by serjeanty of his office as marshal and possibly Hampstead in Berkshire. The "Cartae Baronnum" shows him holding Wigan in Oxfordshire, and Inkberrow in Worcestershire may have been originally John fitz Gilbert's. John was still a minor baron in comparison to the great magnates, but he had increased the inheritance left to him by his father by a great deal.

John fitz Gilbert was a clever and ruthless baron who had more than his share of daring, energy, and ambition. He was known for his ability as a soldier/knight and for his cunning and love of military stratagems. The "Gesta Stephani" describes him as "a limb of hell and the root of all evil." It accuses John of building adulterine castles [probably Newbury in Berkshire], taking the lands of both laity and clergy, and of forcing payments from the church. He put aside his first wife without a qualm in order to better himself and his position. In "L'Historie de Guillaume le Marechal", the chanson de geste written as a history of the life of John's son William, there is a story told of the siege of John's castle of Newbury by King Stephen in 1152. King Stephen held John's son William as hostage for his father's good behavior during a granted truce. John ignored the truce; he used the time to re-fortify and supply his castle. When King Stephen called John to the castle's walls and reminded John that his son's life was forfeit for John's own actions, John said that, "he had the anvils and the hammer to forge still better sons." This was a ruthless warrior and only the gentle nature of King Stephen protected the life of the five-year old William.

John fitz Gilbert died 1164/1165 while his son William was in Normandy being trained as a squire by his cousin William de Tancarville Chamberlain of Normandy. Of the two sons by John's first marriage, the oldest, Gilbert, died within a year of John's own death, and the youngest, Walter, died before John. By the lady Sibile [Sibyl], John had four sons and two daughters; John as the eldest son inherited his father's lands and the office of marshal. John fitz Gilbert's second son, William Marshal, would inherit nothing tangible from his father, but he would be heir to his father's standing in respect to the confidence and favor of King Henry II. John fitz Gilbert, unlike others in the wars between King Stephen and the Empress, changed his allegiance only once. When he joined the Empress' side in the war, he not only served her and her son loyally and faithfully, but he placed his own life in jeopardy protecting and defending her. This was a debt that Henry II remembered and paid. John's son William would do the same for King Henry's wife Eleanor near the castle of Lusignan in Poitou at the end of 1167. Two of the Lusignan brothers attacked and killed William's unarmed uncle, Patrick earl of Salisbury, while Patrick, Queen Eleanor, and William were riding near the castle of Lusignan. William was wounded and taken prisoner while defending the Queen's retreat into the castle and trying to avenge his uncle's murder. William might have inherited some of the physical strength and knowledge of military strategy from his father, but as a second son, he would become in his own right and by his own abilities, skills, and sense of honour the best of chivalric knighthood, a "familiaris Regis," the Earl of Pembroke and regent of England.