With Claude Richard he was one of the original organizers of Louis XV's botanic collection at Petit Trianon, an undertaking quickly joined by Bernard de Jussieu. On 30 June 1746, one year after his brother, he also became a member of the Prussian Academy of Sciences. Like his father and his brother before him, Louis-Guillaume became a member of the Académie des sciences on 3 July 1743, and was elected a fellow of the Royal Society on 7 February 1745, of which his brother also was a member. Later research of his on electrical phenomena was concerned with thunderstorms and the ' fair weather condition'. He researched electrical phenomena, sending a current from a Leyden jar through a wire 950 toises (about 1,850 m) long and concluded that electricity propagated 'instantaneously' in the wire. In the same year, he also began working at the hospital of Saint Germain en Laye as a physician. In 1739 he accompanied the expedition of César-François Cassini de Thury and Nicolas Louis de Lacaille to extend the Meridian of Paris and documented mines and the geology and botany along the route. Louis-Guillaume Le Monnier worked in physics, geology, medicine, and botany. Louis-Guillaume's older brother was the astronomer Pierre Charles Le Monnier.

He was born near Vire as the son of Pierre Le Monnier (1675–1757), who was a scientist himself and a member of the French Academy of Sciences.

Louis-Guillaume Le Monnier (sometimes written as Lemonnier) (27 June 1717 – 7 September 1799) was a French natural scientist and contributor to the Encyclopédie ou Dictionnaire raisonné des sciences, des arts et des métiers.

Le Haschisch by Antoine Monnier