Thomas Englefield was the only son of John Englefield Esq. of Englefield House in Berkshire, by his wife, Jane, the daughter of John Milbourne of London. His father died while he was in his teens and his grandfather, Robert Englefield, had him educated in law at the Middle Temple. His grandfather died in 1470 and Thomas inherited the Englefield estate at the age of only about twenty. About ten years later, he married, for the first time, to Margery, the daughter of Richard Danvers of Prescote in Oxfordshire. Together, they had two sons and eight daughters.

The Englefields also held lands in the Welsh Marches and Thomas therefore became JP for Gloucestershire, Herefordshire, Shropshire and Worcestershire in 1493. To these, he added a similar post in Berkshire the following year, and he held all these offices until his death. Being also an MP (probably for Berkshire), he was elected Speaker of House of Commons in 1497. By 1500, he was a bencher of the Middle Temple and, in 1501, he was knighted into the Order of the Bath. An honour which was followed by his appointment onto the Council in the Welsh Marches the following year. Sir Thomas received many Royal commissions throughout his life, including, for several years, the collection of the lay subsidy tax for Berkshire. He was a Justice in Chester and both North and South Wales, and was made a King’s Councillor in 1509. In this year, he acted as an executor to the will of King Henry VII and was one of the committee appointed to determine coronation claims.

With the accession of King Henry VIII, Sir Thomas Englefield became one of the key men of experience upon whom the young monarch relied in the early years of his reign. Thus, he was a natural choice to serve as Speaker – for a second term – in the first Parliament of Henry’s reign. On 23rd January 1510, he made the usual speech professing his unworthiness for the post and this was balanced by another, delivered at the bar of the Lords, on 23rd February following, in which he praised King Henry's gifts of nature, grace and fortune.

Margery having died, Sir Thomas, not long afterwards, married Mary, daughter of Sir John Fortescue of Ponsbourne in Hertfordshire and widow of both John Stonor (d.1499) of Stonor Park (Oxfordshire) and Anthony Fettiplace (d.1510) of Swinbrook (Oxfordshire). She was the sister of the Blessed Adrian Fortescue and, maternally, a cousin to Anne Boleyn’s father, the Earl of Wiltshire & Ormond.