Ratsimamanga was a pacifist and politically active, and during his years of study, he forged close relationships with French intellectual and political circles. While in France, he co-founded the association of Malagasy Students in France and the Democratic Movement for Malagasy Renovation (MDRM) in 1946 with Jacques Rabemananjara, Joseph Raseta and Joseph Ravoahangy Andrianavalona. MDRM led the protests against the bloody repression of the Malagasy Uprising of 1947. However, MDRM was known to be dominated by Hova elites, who had been politically prominent in the former Merina royal court and wanted to regain the political dominance of the Merina upon independence. Jacques Rabemananjara, Joseph Raseta and Joseph Ravoahangy Andrianavalona were later sentenced to life in prison but were granted amnesty in 1958. Ratsimamanga claimed that he was unaware of the uprising and, thus, was not involved. Later in 1949, Ratsimamanga created the Malagasy National Council, a Government in exile. It was a failure.
On 26 August 1948, Ratsimamanga represented Madagascar at the World Congress of Intellectuals in Defence of Peace, which took place between the 25 to 28 August 1948 of August at Wrocław University of Science and Technology, Poland, and played a role in the framing of the communist powers as supporters of peace, and on the opposite side, portraying the West as a threat to peace.
Ratsimamanga was a member of the delegation that negotiated Madagascars independence from France. 77% of Malagasy voted for independence in the 1958 referendum, and after the independence, Ratsimamanga was appointed the Malagasy Republic ambassador to France from 1960 to 1972. After the 1972 Coup détat, on 14 December 1972, he was appointed the first Ambassador of the Malagasy Republic to China and the Soviet Union. He later established embassies in West Germany, North Korea, and Sierra Leone.
Furthermore, Ratsimamanga represents the Malagasy Republic at the European Economic Community, UNESCO, and Food and Agriculture Organization. He also became UNESCO Vice-chairman of the Executive Council.