Leon Philippe Le Métayer-Masselin was born on May 1, 1831, in Guichainville, Normandy, France.

He was a merchant, archaeologist, numismatist, and botanist from Normandy. In 1852, he married Léontine Masselin (1835–1877), the daughter of Jean-Baptiste Masselin, a ribbon manufacturer in Bernay. He added his father-in-law's name to his own. He became interested in archaeology and numismatics in 1856, conducting research on ancient archives and performing excavations. Around 1862, he and his father-in-law commissioned the construction of a private mansion in Bernay, designed by architect Adolphe Bouveault.

Around 1865, he moved to the Paris region, where he presented himself as a paleographer, archivist, and numismatist. In 1867, he sold a library of over 2,000 books in Paris. He became a member of various organizations, including the French Archaeological Society for the Conservation of Historic Monuments, the Society of Antiquaries of Normandy, the Free Society of Agriculture, Sciences, Arts, and Belles-Lettres of Eure, and the Academy of Cherbourg (National Academic Society of Cherbourg). He served as an inspector for the Normandy Association in the canton of Bernay and was also a member of the Commission for the Topography of Gaul. In 1862, he received 500 francs from the CTG to excavate in Berthouville (Eure).

In 1874, he emigrated to Canada with his two daughters, Marie and Élisa, and his son Raoul. He claimed that financial promises from the Canadian government and encouragement from the bookseller Bossange lured him there to establish a ribbon factory, initially in Saint-Hyacinthe and later in Chambly. Unfortunately, his venture failed, leading to accumulated debts and prompting him to publish an anticlerical pamphlet.

Around 1879, he began referring to himself as Le Métayer de Guichainville and sought opportunities as a botanical collector at the National Museum of Natural History in Paris. Eventually, he moved to New York, where he worked as a private French tutor and numismatic expert, even selling a coin collection in 1881. Newspaper articles, sometimes sensational in nature, portrayed him as a ruined nobleman, specifically as Baron or Marquis de Guichainville, living in poverty with his children, one of whom was blinded in a traffic accident in 1911.

He died on October 1, 1911, in New York City, New York.