A Neolithic ax jadélite, calens from the site of the luminaire, testify to the ancient history of Lachau.
In 1209, Lachau has a charter of liberties granted by Raybaud Lachau of the son of Ripert Mévouillon. This charter, very liberal for its time, is one of the first in the region.
The Church of the Magdalene, the Priory of the Knights Templar and the house had completely disappeared. All that remains of this period (thirteenth century) that the Church of Our Lady of Calma and around the variable.
The lands of the nobles and clergy were seized and sold, and the castle, between 29 and 24 Ventose third Germinal (March 8, 1795, April 2, 1795). The current land register is still marked by land fragmentation.
Lachau population was about 120 inhabitants in 1428, 400 in 1636, 675 in 1775, 737 in 1840, 907 in 1860, 624 in 1884 and 206 today.