Séguy was born in Toulon, France, on 1 July 1877. He showed a passion for natural history at a young age, and began collecting insects at the age of 12. He studied medicine at the University of Montpellier and graduated in 1902.
Séguy began his career as a military doctor, but he soon devoted himself to entomology. He worked at the Muséum national d'histoire naturelle in Paris from 1906 until his retirement in 1942. During his time at the museum, he curated the collection of Diptera, and published numerous scientific articles and books on the subject.
Séguy's main area of research was the family Muscidae, and he became a world expert on this group. He described over 1,000 new species of Muscidae, and published several monographs and catalogues on the family. He also worked on other families of Diptera, including Tephritidae, Conopidae, and Cecidomyiidae.
Besides his scientific work, Séguy was also an active member of several scientific societies, including the Société entomologique de France and the Royal Entomological Society of London. He was awarded the Linnean Medal of the Linnean Society of London in 1939, and the Prix Constant from the Académie des sciences in Paris in 1949.
Séguy died in Paris on 23 October 1951, at the age of 74. He was a prolific entomologist who made significant contributions to the study of Diptera, and his work is still widely cited by scientists today.