Hermann Otto Monnig (or Moennig) studied at Victoria College, Stellenbosch (soon to become the University of Stellenbosch), and was awarded the BA degree with honours in Zoology by the University of the Cape of Good Hope in 1917. He continued his studies in zoology and parasitology in Amsterdam and then Zuerich, where he was awarded a DPhil in zoology in 1921 for a thesis on the histology of Trematodes. On his return to South Africa in 1922 he was appointed research officer (parasitology) at the Onderstepoort Veterinary Research Institute and obtained his BVSc in 1926 at the Transvaal University College (soon to become the University of Pretoria). Three of his early papers, all published in the Transactions of the Royal Society of South Africa, were "On some new South African parasitic Nematodes" (1924), "A new Trichostrongylus from South African sheep" (1925), and "On a new Physaloptera from an eagle and a Trichostrongylus from the cane rat..." (1927). In 1928, following the departure of the helminthologist Dr F. Veglia*, he was appointed head of the Section Parasitology, and in 1929 professor of parasitology, at the Onderstepoort Faculty of Veterinary Science. During the next 16 years he published many more papers on his research in parasitology and chemotherapy for parasitic diseases. Altogether he described some 46 new helminths found in domestic and wild animals. In 1934 the first edition of his classic textbook Veterinary helminthology and entomology, which subsequently became the international standard textbook for tuition of these subjects, was published in London.
In 1945 Moennig resigned from both his posts and founded a commercial company, Agricura Laboratories, to manufacture remedies for the control of parasitic infestations and related diseases of livestock. During his lifetime many honours and distinctions were bestowed on him: The Senior Captain Scott Medal of the South African Biological Society (1945); the Havenga Prize (1947), honorary membership (1969) and the M.T. Steyn Prize (1971) of the Suid-Afrikaanse Akademie vir Wetenskap en Kuns; DSc (honoris causa) degrees by the University of Pretoria (1953) and the University of the Free State (1970); and a Fellowship of the American Association for the Advancement of Science. He also served in various prestigious positions from about 1953 onward: Chairman of the National Parks Board of Trustees, chairman of the council of the University of Pretoria, chairman of the Council for Scientific and Industrial Research, chairman of the Scientific Advisory Board, chief scientific adviser to the Prime Minister, and member of the board of the South African Broadcasting Corporation.
Moennig was author or co-author of at least 70 scientific publications up to 1947 and authored two popular scientific books in Afrikaans: Parasiete en parasietiese siektes van die mens (Johannesburg, 1944) and (with F.J. Veldman), Handboek oor veesiektes (Cape Town, 1954).