Thomas was the only child of the Judge and Baron of the Exchequer, Sir John Foxley of Foxley Manor  in Bray (Berkshire) and Bramshill Manor in Eversley (Hampshire), by his wife, Constance, probably the heiress of the De Bramshill family. He married at a relatively young age to Katherine, the daughter and co-heiress of Sir John Ifield of Apuldrefield in Cudham in Kent and Swallowfield in Berkshire. The couple brought at least one son and four daughters into the World while Thomas was, no doubt, found minor positions at court by his father. It was not until two years after Sir John’s death, in 1325, that Thomas was elected to represent Berkshire in Parliament. That same year, King Edward II was deposed and, in 1328, Thomas took the dangerous step of accepting the Constableship of Windsor Castle under the patronage of the new regime headed by Queen Isabella and her lover, Mortimer.

Perhaps the new nominal monarch, Edward III, noted some instances of kindness from Foxley, who the lad must have seen often at Windsor. For, upon taking full control of the country in 1330, the young king allowed Thomas to keep his offices. While his mother remained at Foxley, Thomas and his wife had moved into the old house at Bramshill. Although it was for Berkshire that he was again returned as MP in 1332, since he spent most of his time in Windsor. It was, no doubt, at the Royal Castle that Thomas' son, John, first became acquainted with Matilda, the daughter of the Master of the King's Horse and his father's good friend, Sir John Brocas. The two caused their parents some anxiety by apparently running away together at the age of only fourteen and persuading the Vicar of Bray to marry them!

In 1333, Constance Foxley passed away and a home much more convenient to Windsor was therefore vacated. Thomas' growing family almost certainly relocated to Bray, from where he was, once more, MP for Berkshire in 1337. Certainly by 1341, they were being granted permission to hear divine service in their chapel there. Not long afterwards, Katherine must have died, for Thomas remarried some time after 1342 to Joan, the widow of James Woodstock.

Thomas Foxley was constable at Windsor through some of the highest points in the castle’s long history. He was there for the great tournament of 1344 at which King Edward promised to instigate a grand order of knights in honour of St. George; at the famous legendary ball, two years later, at which the Fair Maid of Kent dropped her garter; and, in 1348, when the Order of the Garter was formally inaugurated. Just prior to this last event, Queen Philippa was at the castle for the birth of her fourth son, William. The captive King David II of Scots joined the celebratory feasts and remained at Windsor for the next eleven years. In the meantime, there had been vast movements of troops during the Crecy campaign against France and, in 1347, Thomas’ own Berkshire estates were charged with the supply of 120 archers for the (Hundred Years’) War.

With the establishment of the Order of the Garter and an established truce in Europe, Windsor thronged with foreign knights and King Edward decided that the castle needed considerably updating if it was to be the home of English chivalry. Furthermore King David was complaining about the state of his accommodation. A young architect named William of Wykeham appears to have been recommended by the Bishop of Winchester and hundreds of workmen arrived in Windsor to almost entirely rebuild the fortress. Thomas found that he had many more responsibilities than those usually asked of the constable, so, in 1351, the King commissioned his friends, Sir John Brocas and Oliver de Bordeaux to help him oversee the project and "survey the workmen and their work, to encourage such as did their duty, but to compel those who were slothful". At the same time, Thomas had to contend with large numbers of pilgrims visiting the sacred 'Cross of Gneth' held at St. George's Chapel. When the war recommenced however, Sir John was recalled to France and a subsequent commission to complete the work was given to Wykeham in 1356. Of course, this man eventually rose to fill his original patron’s post as Bishop of Winchester and he always remained a firm friend of Foxley, whom he selected as one of three persons for whose benefit mass was daily to be said in the chapels of his foundations, Winchester and New College, Oxford.

During this period of rebuilding at Windsor, Thomas appears to have taken no little advantage of the architectural expertise available to him there. As early as 1347, he was granted a licence to enclose a (still extant) park of 2,500 acres at Bramshill and subsequently he seems to have decided to rebuilt his house there too, as a more castle-like structure. Within the present day Jacobean mansion, considerable remains of this early fortress still exist, erected in the style of the period, round a court 100 feet long by 80 broad, with walls of great thickness, the vaulting of the cellars and other parts being precisely similar to what may be observed in the steward's room and servants' hall at Windsor Castle. The workmen employed by Foxley at the Royal Palace were, presumably, the same as those engaged in the erection of Bramshill.

The last years of Thomas' constableship were largely taken up with the keeping an eye on two new Royal prisoners, King John II of France and his son, Philip, both captured at the Battle of Poitiers in 1356. He died in 1360 and was buried in All Saints’ Chapel in Bray Church, where his son erected a, now lost, marble monument with engraved brass effigies. He had held the position of Constable of Windsor Castle for thirty-three years. He was also steward of the manor of Bray. His manors of Bramshill and Foxley, as well as other lands in Bray, White Waltham and Binfield, were inherited by his son, Sir John Foxley.