In the late 1950s he became a leader in Patrice Lumumba's Mouvement National Congolais (MNC) party and worked to expand its influence.

[1] His family was part of the Kusu tribe of the Tetela ethnic group[2] and came from the village of Lukonge, Aluba chieftaincy, Kibombo Territory, Maniema, Belgian Congo.

[3] Due to his family's Muslim background, Kasongo was a firm supporter of anti-clericalism and opposed the Catholic Church's political influence in the Congo.

[8] The following year Kasongo's friend, Patrice Lumumba, created the Mouvement National Congolais (MNC), a nationalist organisation, and tasked him and several others to form a central committee.

[3] The civil unrest that had surfaced during the year caused the Belgian government to arrange a round table conference in Brussels to discuss the political future of the Congo.

Kasongo was invited to attend as a delegate for the MNC but threatened to boycott the conference unless Lumumba, who had been arrested and imprisoned, was allowed to go as well.

[14] In the May elections Kasongo won a seat in the Chamber of Deputies on an MNC ticket as a representative from the Haut-Congo constituency with 2,429 preferential votes.

[26] In October Kasongo was made a member of a commission assembled by Lumumba[d] tasked with managing his relations with the United Nations Operation in the Congo.

[30] However, by January 1961 Mobutu's government of commissioners was foundering due to financial problems and Kasongo angrily demanded that Lumumba be restored to the premiership.

When proposals were made to have the dispute over governance settled in a round table discussion, Kasongo rejected the idea and demanded for Parliament's powers to be restored.

If ensured real security in its work, the Congolese Parliament members this time will once again raise their weighty voices in defence of the republic, its unity and its indivisibility."

In mid-February the United Nations (UN) established a program by which potential political targets could seek military protection in designated facilities.

[34] Deputy Prime Minister Bolikango personally requested that he return to his residence under government protection, but Kasongo chose to remain with his family in a UN-guarded facility.

[38] On 17 April 1963 Kasongo was made Deputy Prime Minister with responsibility for the Economic Coordination portfolio in Cyrille Adoula's new government.