On qualifying in 1882, his rather quiet and studious life was changed by an invitation to accompany the botanist Émile Bescherelle (1828-1903) on an expedition to Cape Horn aboard the ship La Romanche, a trip during which he amassed a fine collection of algae.
On returning to France he studied natural sciences and was awarded a degree in 1888 whereupon he joined the National Museum of Natural History in Paris, working in the cryptogamic herbarium under Professor van Tieghem.
He revised when necessary the taxonomy and systematics of the species described, and his name appears frequently as a second author for many species of algae.
In 1890 he was awarded the Prix Montagne by the Academy of Sciences for his monograph on terrestrial algae of the genus Trentepohlia.
He was awarded the Prix Desmazières for 1913 for his work on marine flora.Besides publishing a guide to phycology in 1892, he contributed to a number of illustrated books on plants, and published the first practical guide to seaweeds from the coast of France.
Hariot's main interest was in algae and fungi, and later including the rusts.