Ratsimamanga started working at the French National Centre for Scientific Research (CNRS) in 1945 after he was approached by Frédéric Joliot-Curie, CNRS's research director and Nobel prize laureate in Chemistry (1935). At CNRS, he pioneered the study of Human blood group systems, and treatments for leprosy and tuberculosis. Ratsimamanga work showed the presence of hormones in the diet and their role in the development of the body, while eliminating the factors of cellular detoxification, especially in the liver.
Ratsimamanga was the founding director of the  (IMRA) in 1957. IMRA was focused on Phytotherapy to use local plants and traditional practices to cure diseases, i.e., traditional pharmacopoeia. IMRA succeeded in using the Syzygium cumini tree as an anti-diabetic agent, and creating alternative medicines against malaria, leprosy, asthma, lithiasis, blood pressure, hepatitis and other common conditions.
Ratsimamanga was the head of Malagasy National Academy, and a Professor Emeritus of the Faculty of Medicine, University of Antananarivo. He was one of the founders of the World Academy of Sciences in 1983, and the African Academy of Sciences in 1985. He was a member of the Royal Academy for Overseas Sciences, Institut de France (1966), and the Académie Nationale de Médecine (1967).