Jacques-David LeCoultre (1875–1948) was a grandson of Antoine LeCoultre, who founded the watchmaking business at Le Sentier in the Vallée de Joux in 1833. According to German Wikipedia he became head of watch production at LeCoultre et Cie in 1900 and general director in 1906, and he led the manufacture until his death.
In 1903 the Paris watchmaker Edmond Jaeger, supplier to the French Navy and to Cartier, challenged Swiss manufacturers to make ultra-thin movements. LeCoultre took up the challenge with a series of extra-flat pocket watches, among them, in 1907, the thinnest in the world, with the LeCoultre Calibre 145 just 1.38 mm thick. The collaboration with Jaeger produced the Duoplan of 1925 to 1929, a wristwatch movement laid out on two levels to allow a larger balance, which German Wikipedia calls the first wristwatch with sapphire glass. Jaeger also bought the patent for the Atmos clock, driven by changes in air temperature, from its inventor Jean-Léon Reutter and licensed it to LeCoultre from 1936.
In 1931 the watch dealer César de Trey wrote to him on behalf of polo players in British India who wanted a watch that could survive a match. LeCoultre worked with the French engineer René-Alfred Chauvot, who patented a rectangular case that slides and flips over in its carriage to protect the dial, and the Reverso was born. In 1937 the partnership with Jaeger became a merger, and the company took the name Jaeger-LeCoultre. Under his direction it took a stake in Vacheron & Constantin in 1938, doubled its turnover between 1937 and 1946, and came through the Second World War unscathed by diversifying its production. He was chairman of the board when he died in 1948; Georges Ketterer succeeded him.
The date of the Jaeger and LeCoultre merger is given as 1937 by French and German Wikipedia, while another German passage dates the joint name to 1930.