John was the son of John Kentwood Senior, the Lord of the Manor of Kentwood at Tilehurst in Berkshire.  At a young age, he became a soldier, accompanying the Black Prince on his expedition to Gascony in 1355. It was the era of the Hundred Years' War and King Edward III was busy pressing his claim to the French Throne. At the great English victory at the Battle of Poitiers, Kentwood proved his worth by capturing Prince Philip of France, with the help of Sir Edmund Wauncey.

The Black Prince purchased the prisoner from Kentwood for 4,000 marks (£2,666-13s-4d) which he paid him in varying amounts over a long period. By 1362, this had settled at £100 per year and it was increased by £33-6s-8d two years later in return for which John agreed to rejoin the Prince fighting in Gascony within eighteen months. He almost certainly continued to campaign with his patron for the next four or five years and was knighted and awarded a £40 annuity for his military service on 22nd January 1370. Sir John may have recouped all his money by this time, for he immediately began acquiring land throughout Southern England which had belonged his wife's previous husband.

From around 1375, Sir John became highly involved in the local administration of his home county of Berkshire, being its MP in three parliaments. The following year, in partnership with John De La Mare, he was sent to affect the capture of a notorious 'sorcerer'. A certain Dominican Friar was widely suspected of having used magic to enable the the aging King's mistress, Alice Perrers, to assert undue influence over him. The two Johns disguised themselves in order to gain entry to the friar's house and promptly arrested him. Prince John of Gaunt subsequently had Perrers banished from the Edward III's presence.

With the accession of King Richard II, Sir John Kentwood was appointed Steward of the Duchy of Cornwall in the Autumn of 1378, a post which took him far from both home and the Royal Court. His Cornish headquarters was at Restormel Castle, near Lostwithiel, and from there he was kept exceptionally busy, sitting as a JP and taking on enumerable commissions. However, Sir John was also employed as a Royal emissary. In the Spring of 1381, he spent two months in Brittany, supervising the Earl of Buckingham's forces there. Upon his return, he reviewed the Earl of Cambridge's army at Dartmouth and Plymouth before they set sail for Portugal. In 1383, he supervised the muster of the retinue of William Le Scrope, Seneschal of Acquitaine, and, two years later, he did the same for the Chancellor of Portugal. He was given custody the South of England during King Richard's absence in the North and was later commissioned to advance the embarkation of John of Gaunt's army for Castile, when he was pressing his claim to the throne there in 1386. Then, in 1388, he was appointed to survey the estates of the Lords Appellant victims of the Merciless Parliament. Yet the Lords Appellant still removed him from office later the same year.

Despite his long and loyal service to the Crown, Sir John seems to have been little favoured by King Richard. He was kept far from the centre of government and received little recompense for his hard work. His extensive legal experience made him an ideal candidate for the appointment of Judiciar of South Wales in 1389. Further commissions took him largely to Yorkshire and the North of England, particularly in connection with the administration of the estates of Queen Anne. In 1390, he advised the King's Council on the situation in Gascony, but his official duties were now much reduced. Sir John died four years later. His wife, Alice, the daughter of Gerard Braybrooke of Colmworth in Bedfordshire and Horsenden in Buckinghamshire, outlived him by at least ten years. His estates were inherited by his eldest son and namesake, John. His more famous younger son, Reginald, was to become Dean of St. Paul's in 1422.