Thomas de Havilland (alt surname spelled "de Haveilland"), son of __?__ & __?__, was born ca 1412 (estimated herein).
There are no known records of his parents, and thus an unbroken ancestral lineage is speculation until more evidence can be uncovered. 
While his birthdate is not known, it is guessed to be sometime between 1410-1420. Evidence of the name of his spouse and mother of his children are not to be found at this time, and is entirely speculative at this point, with marriage into the de Sausmarez family a healthy guess only given the relationship between the two families for many generations thereafter. 
Thomas married __?__. Supposition that he married a one "Marguerite de Sausmarez" is not yet supported by any primary (or even secondary) evidence and must be considered fictional for now.
Thomas, Sieur de Havilland (it was custom amongst them to place "Sieur" after the given name and before the surname), resided in Guernsey and descended from a family of knights going back many generations and probably originating in ancient Neustria. Some of his ancestors might have participated in the crusades, and might have been Templars (of the original order), but evidence is scant and ambiguous.  His ancestors were subjects of the Merovingian Franks and were wealthy fief lords there until the Norman conquest.  The Y-DNA haplogroup of his paternal descendants who trace themselves genealogically back to Thomas help to validate these origins. 
Thomas became a Jurat of Guernsey in 1470, according to the Visitation of Gloucestershire in 1623, as reported by John V. S. de Havilland.  An alternative source claims he was elected in 1472 (according to Richard Hocart, who might have been referencing the work of de Lacy Mann).  (A Jurat is a magistrate in the Channel Isles who is generally a life member of the Royal Court of Justice. The Jurats still reside. Their influence as heads of state are akin to a mix between Judiciary and Legislative powers, outranked only by a Bailiff on the island of Guernsey, pledged to the royalty of Great Britain. ) Thomas was largely responsible for the establishment of the knitting industry of that island; obtaining a royal grant to import wool and cloth from England and re-export it to Normandy and Spain.
Castle Cornet at St. Peter Port's harbor, Guernsey Seat of Guernsey government, including all the de Havilland Jurats, until 1672
The Heraldic achievement of Thomas de Havilland was like that of his ancestors: a black castle triple-towered on a silver shield (differenced in some aspect as per custom). But on his coat of Arms Thomas was bestowed in 1467 what might be the first crest in the family (that is the adornment over the shield), awarded for his "distinguished gallantry at the recovery from the French of Mont Orgueil Castle, in Jersey."  (Also see The de Havilland Bearings.)
The castle of Mont Orgueil, the chief stronghold defending the island of Guernsey against the Normans, fell into the hands of a Norman chieftain, Surduval, when the then governor of Orgueil and Jersey, Naufan, surrendered the castle in a scheme of scandalous treachery. The French claimed to possess Jersey while conflict with local rebellions ensued for six years. Finally, in 1467, Admiral Sir Richard Harliston from Guernsey led Thomas de Havilland and a huge fleet of ships into a siege of the castle which lasted six months. The castle was forcibly recovered from the French. It was for this event that his achievement received a crest, and he became (like his forefathers and other family) a Jurat of Guernsey in 1470. 
Mont Orgueil Castle, Jersey, Channel Islands
In order to avail himself of the privileges the Patent of Edward IV conferred upon him, due to this milestone, he established his son James as Mayor of Poole, Dorsetshire, England, in 1471. 
It has been argued that James was therefore the "founder of the family in England," however some professional genealogists hesitate at this conclusion as it has no basis in hard evidence. Genealogically, James' descendants did leave a stronger footprint in the England record, and from them appear to have descended the Haviland family of America and Canada.
The de Havilland sons and grandsons established the high stature of the family in England with a succession of Mayors in Poole especially, and a few in Salisbury, and this may have strengthened genealogical records such that Thomas of Guernsey is now the earliest traceable ancestor in the family, despite dozens of other de Haveillands, de Havyllandes, and others on Guernsey well before Thomas, whose interrelationships are lost.
Father of the Genealogy
Thomas de Havilland is the progenitor of a majority of the population today who bears the name de Havilland, Haviland, Heavilon, Heavirland, and Heavilin. This has been dubbed "The Guernsey Family," so as to differentiate it from "The Irish Family" who might be found with the surname Haviland but trace back to Havlin or other spellings, and "The German Families" which Y-DNA testing is proving to be a collection of diverse genealogies with the name Haverland who were probably geographically named (from Dutch: "oat"=haver + "land"=field), some of whom may today also use the spelling Haviland. While some ancient members of the Guernsey Family have been popularized with the spelling "Haverland," this is a misleading application, being that the original records may not have spelled it that way, and thus re-translated it has caused some confusion now that we know there is a different family of that spelling that originated in Germany. It is likely that most of the primary evidence which John V. S. de Havilland in The Chronicle de Havilland referred to as "Haverland" (and Thomas Fiott de Havilland before him), and thus which appears in Burke's Peerage which their work influenced, did not in fact spell the name that way. It was more likely in ancient times spelled de Havilant or d'Avilant.
It may be possible to confirm family relationships with Thomas by comparing test results with other carriers of his Y-chromosome or his mother's mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree: