Mwamba was appointed to serve as Minister of Justice in the first government of the independent Congo under Prime Minister Patrice Lumumba. The government was invested by Parliament on 24 June 1960. He competed with Minister of Interior Christophe Gbenye to assert his authority over the Sûreté Nationale (security police) until Lumumba attached the chief of the organisation to his own office. On 5 July the Congolese garrisons in Léopoldville and Thysville mutinied, triggering a domestic crisis. On 8 July the Council of Ministers convened to discuss reorganisation of the army. Several ministers wanted Joseph-Désiré Mobutu to become the new Commander-in-Chief. Mwamba believed he was too young and suggested Victor Lundula for the role. The ministers eventually compromised, accepting Mwamba's recommendation and making Mobutu the army chief of staff. Three days later Mwamba ordered the Procureur général to launch inquiries into the actions of soldiers against Europeans in the province of Kasai. On 28 July he was made a member of a cabinet committee established to coordinate government actions with those of United Nations officials. In early August he went to New York as part of a Congolese delegation to address the UN Security Council. On 5 September President Joseph Kasa-Vubu dismissed Lumumba, Mwamba, and several other members of the government from office.