After graduating in medicine (1879), Ross entered the Indian Medical Service and served in the third Anglo-Burmese War (1885). On leave he studied bacteriology in London (1888–89) and then returned to India, where, prompted by Patrick Manson's (Manson, Sir Patrick) guidance and assistance, he began (1895) a series of investigations on malaria. He discovered the presence of the malarial parasite within the Anopheles mosquito in 1897. Using birds that were sick with malaria, he was soon able to ascertain the entire life cycle of the malarial parasite, including its presence in the mosquito's salivary glands. He demonstrated that malaria is transmitted from infected birds to healthy ones by the bite of a mosquito, a finding that suggested the disease's mode of transmission to humans.

Ross returned to England in 1899 and joined the Liverpool School of Tropical Medicine. He was knighted in 1911. In 1912 he became physician for tropical diseases at King's College Hospital, London, and later director of the Ross Institute and Hospital for Tropical Diseases, founded in his honour. In addition to mathematical papers, poems, and fictional works, he wrote The Prevention of Malaria (1910).