In 1938 Nkumbula joined the Northern Rhodesian government's teaching service and later worked in Kitwe and Mufulira on the Copperbelt. During World War II he became involved in African nationalist politics, like many other educated Africans of the day. For example, he held the position of Secretary of the Mufulira Welfare Association and co-founded the Kitwe African Society.
In 1946, from Chalimbana Teacher Training School, Nkumbula went to Kampala's Makerere University College in Uganda. This was made possible by the support of Sir Stewart Gore-Browne, a pro-Black British settler politician. From Makerere, Nkumbula went on to study for and received a diploma from the Institute of Education, University of London. In London, Nkumbula had the opportunity to meet other African nationalists who were galvanized after attending the 1945 Pan-African Congress in Manchester, England. In 1949 he worked with Nyasaland's Hastings Kamuzu Banda in drafting a document that expressed African opposition to the proposed White-dominated Central African Federation. This collaboration prepared the two men for their subsequent struggles with the colonialists in their home countries. After his diploma, Nkumbula enrolled to study economics at the London School of Economics but he failed his examinations and returned to Northern Rhodesia without a degree early in 1950.
In early 1955 Nkumbula and Kaunda were imprisoned together for two months (with hard labour) for distributing "subversive" literature. Such imprisonment and other forms of harassment were normal rites of passage for African nationalist leaders. The experience of imprisonment had a moderating influence on Nkumbula, but it had a radicalizing influence on Kaunda. Nkumbula became increasingly influenced by White liberals and was seen as willing to compromise on the fundamental issue of majority rule.
Opposition to Nkumbula's allegedly autocratic leadership of the ANC eventually resulted in a split with Kaunda, who went on to form the Zambia African National Congress (ZANC) in October 1958. ZANC was banned in March 1959 and in June Kaunda was sentenced to nine months imprisonment. While Kaunda was still in prison, the United National Independence Party (UNIP) was formed late in 1959. Once he came out of prison, Kaunda took over the presidency of UNIP, which became better organized and more militant than Nkumbula's ANC. Due to this, UNIP rapidly took the leading position in the struggle for independence, eclipsing the ANC.
During independence constitutional talks in London in 1960–61, Nkumbula played only a secondary role. He suffered a further setback when he disappeared from the political scene for nine months (April 1961 - January 1962), while serving a prison sentence for causing death by dangerous driving. In the run-up to elections in October 1962, Nkumbula made the mistake of accepting funding from Moise Tshombe's regime in Katanga. He also made an ill-advised secret electoral pact with the Whites-only United Federal Party (UFP). He then found himself in a bind after the ANC won seven seats and held the balance of power between UNIP and the UFP. Eventually Nkumbula chose to form a coalition with UNIP and was given the post of minister of African education. The UNIP/ANC alliance lasted until the pre-independence elections of January 1964, when UNIP won fifty-five seats to the ANC's ten seats. Nkumbula became leader of the opposition.
Nkumbula died on 8 October 1983.
His son Baldwin Nkumbula was also a politician. He was widely tipped to become the next president of Zambia until his life was tragically cut short in a road accident.
D. Mulford, Zambia: the politics of independence (1967)
G. Mwangilwa, Harry Mwaanga Nkumbula: a biography of the ‘old lion’ of Zambia (Lusaka, Zambia, 1982)
K. Macpherson, Kenneth Kaunda of Zambia: the times and the man (1974)
J. J. Grotpeter, B. V. Siegel, and J. R. Pletcher, Historical dictionary of Zambia, 2nd edn (1998)