Uruguayan ornithologist and botanist. Born in Montevideo his father was an astronomer who had received a good education in France. Legrand was brought up to appreciate the sciences and arts, especially European literature, and often travelled with his parents to Europe where he studied at the College of Laussane in Switzerland between (1912-1914). His initial vocation was as a critic of art and literature, publishing his reviews and his own poetry in many journals during the 1920s. In 1929 he married Elba Castellanos Regules and together they would have a son, also named Diego. Legrand was an accomplished artist and his first publication in the area of natural sciences was the illustration of fish species for volumes 3 and 4 of the "Album Ictiológico del Uruguay" in 1936 and 1940 respectively. He made many important connections within the Institute of Natural Sciences and published his first academic work in El Hornero (the publication of the Ornithological Society of La Plata) in 1926; it was entitled "Nomenclatura Bastarda" and analysed common names of species used in Europe that corresponded to different species in the countries of La Plata. This publication focused largely on bird species, perhaps due to his previous curation of two large aviaries, and he went on to publish on the management of captive bird species (1934). For this article he obtained the help of the German botanist Dr. Herter to classify certain tree species and quickly became a student of Herter in the study of plants, a field he would remain in for the rest of his life. He began by undertaking excursions in the areas surrounding Montevideo, especially in Carrasco and soon published the "Comunidades Psamófilas de Carrasco (Uruguay)". At this time he entered the recently created Linnean Society and spent a lot of time in the Museum of Natural History where he met another German Botanist, Cornelio Osten, who was an important influence on his career in botany.

Between 1938 and 1951 Legrand was sub-director of the Museum and spent his time adding to the herbarium collections, before focusing on the genus Portulaca, and finally the Myrtaceae of tropical America. He then became the director of the Museum and looked after the publications created by the institute along with two others, the "Flora Ilustrada de Uruguay" and "Comunicaciones Antropológicas". In 1958 the French government named him a gentleman of the Legion of Honour and in 1972 he was designated vice-president of the First Latin American Botanical Congress. He retired in 1970 and was made an associate researcher, remaining in the museum to study until he died, by which point he was responsible for over 80 publications and for creating 328 plant names new to science.