Antoine Laumet, born to a local magistrate…

Antoine Laumet, born to a local magistrate at Les Laumets in the hamlet of Saint-Nicolas-de-la-Grave, in Gascony, and educated in a military school, had joined the army where he was a cadet in the regiment of Dampierre-Lorraine, and in 1677 had become a lieutenant in the regiment of Clairembault.

He arrived in 1683 at Port-Royal, Acadia, where Governor Frontenac made him a lieutenant and later a captain.

Soon after disembarking the colorful Laumet  had changed his name and began calling himself "Lamothe", which was the name of a nobleman of his home region (de Lamothe-Bardigues) who was a counselor to the parliament in Toulouse.

He also borrowed the latter's coat of arms that he had seen on the gates of the Bardigues estate near St. Nicolas. (It is a variation on that coat of arms that has graced the Cadillac automobile for close to one hundred years.)

He married Marie-Thérèse Guyon, niece of French-Canadian privateer, Denis Guyon, in 1687.

It is rumored that they met at the Governor's ball at Quebec's Château St. Louis and that Laumet worked for her uncle as a navigator.

He signed the marriage license de La Mothe, sieur de Cadillac.

The style "sieur" is derived from the French formal address "monsieur," and equates to the English title "sir"; it is roughly equivalent to a title between "gentleman" and "lord."

It is believed that the name Cadillac is a reference to a hamlet near the town of Montech, not far from Laumet's birthplace, or perhaps the wine-producing Cadillac, seat of the Ducs d'Épernon, which is also close to Gascony. (The couple will eventually have between seven and thirteen children.)

Cadillac in 1688 had requested and received from the Governor of New France a parcel of land in an area known as Donaquec which included part of the Donaquec River (now the Union River) and the island of Mount Desert in the present-day U.S. state of Maine.

Cadillac at that time referred to himself as Antoine de la Mothe, Sieur de Cadillac, Donaquec and Mount Desert.

He had gone to France in 1689 to consult on the war effort against New England, then moved with his family in 1691 to Quebec City, where he had been commissioned in the Troupes de la Marine.

He is named in 1694 as commander of the strategic western fort of Michilimackinac in Michigan.