Taking part in the Egyptian Government's helminthological survey in Uganda in 1906/07 he described several new species of intestinal nematodes in elephants. In 1909 he identified the parasite, Trichostrongylus pergracilis, as the cause of Grouse Disease.
His many important contributions included the incrimination of Chrysops in the transmission of Loa loa, the development of S. mansoni in Bulnius snails and S. haematobium in Planorbis and the mode of infection by cercariae boring through the skin and, with Patrick Manson, the life cycle of Dracunculus medinensis in its copepod host. Leiper also conclusively showed experimentally that Schistosoma mansoni and Schistosoma haematobium were distinct species.
He was instrumental in initiating schemes to control schistosomiasis (snail fever) and dracunculiasis (guinea worm disease)and advocated the need for supplies of fresh water to prevent water-borne parasitic infections. Robert Leiper also made lasting contributions to our understanding of the biology and taxonomy of species infecting humans belonging to the genera Opisthorichis (Clonorchis), Metagonimus, Echinochasmus, Metorchis, Fasciololpsis, Necator, Ancylostoma, Enterobius, Onchocerca, Mansonella, Gnathostoma and larval tapeworms as well as descriptions of other worms including potato eelworm.