De Lessert was born in
Lavigny, Canton of Vaud, Switzerland in the noble family of Henry de Lessert and Marie Tronchin de la Rive. He studied at the University of Geneva and received a doctorate in 1903 under Emile Yung (1854–1918) who succeeded
Carl Vogt. In his thesis he introduced 60 species of spiders new to Switzerland. In 1906 he married his cousin Jacqueline de Lessert de Neufville. He was active in the Zoological Society of Switzerland and contributed to the catalogue of invertebrates of Switzerland by
Maurice Bedot. In 1908 he joined the
Museum of Natural History in Geneva to work on the arthropod collections. He retired in 1918 and moved to
Buchillon where he spent most of his time studying spiders. In 1908 he made a trip to East Africa on a collecting expedition as a substitute for Jean Carl (1877–1944). He later studied the spiders collected on Swedish expeditions by
Yngve Sjöstedt and
A. Tullgren as well as by the American Museum to Uganda, Kilimanjaro, South Africa, Congo, Angola, and Portuguese East Africa between 1913 and 1922. A street in Buchillon is named after him as Rue Roger-de-Lessert. == References ==