After suspension from college, Mandela went to Johannesburg where he completed his BA by correspondence, took articles of clerkship and commenced study for his LLB. While studying in Johannesburg he entered politics in earnest by joining the African National Congress in 1942. At the height of World War II a small group of young Africans, members of the African National Congress, banded together under the leadership of Anton Lembede. Among them was William Nkomo, Walter Sisulu, Oliver R. Tambo, Ashby P. Mda and Nelson Mandela. These young people set themselves the formidable task of transforming the ANC into a mass movement, deriving its strength and motivation from the unlettered millions of working people in the towns and countryside, the peasants in the rural areas and the professionals.  Their chief contention was that the political tactics of the 'old guard' leadership of the ANC, reared in the tradition of constitutionals and polite petitioning of the government of the day, were proving inadequate for the tasks of national emancipation. In opposition to the 'old guard', Lembede and his colleagues espoused a radical African Nationalism grounded in the principle of national self-determination. In September 1944 they came together to found the African National Congress Youth League (ANCYL).

 Mandela soon impressed his peers by his disciplined work and consistent effort; he soon rose through the ranks of the organization and was elected to the Secretaryship of theYouth League in 1947. By painstaking work, campaigning at the grassroots and through its mouthpiece Inyaniso' (Truth) the ANCYL was able to canvass support for its policies amongst ANC membership. Spurred on by the victory of the National Party which won the 1948 all-White elections on the platform of Apartheid, at the 1949 annual conference, the Program of Action, inspired by the Youth League, which advocated the weapons of boycott, strike, civil disobedience and non-co-operation was accepted as official ANC policy.

 A sub-committee of the ANCYL, which comprised of David Bopape, Ashby Mda, Nelson Mandela, James Njongwe, Walter Sisulu and Oliver Tambo, had drawn up the Program of Action. To ensure its implementation the membership replaced older leaders with a number of younger men. Walter Sisulu, a founding member of the Youth League was elected Secretary-General. The conservative Dr A.B. Xuma lost the presidency to Dr J.S. Moroka, a man with a reputation for greater militancy. The following year, 1950, at the national conference Mandela was elected to the NEC.

 The ANCYL program was aimed at the attainment of full citizenship and direct parliamentary representation for all South Africans. In the policy documents of which Mandela was an important co-author, the ANCYL paid special attention to the redistribution of the land, trade union rights, education and culture. The ANCYL also wanted free and compulsory education for all children, as well as mass education for adults. When the ANC launched its Campaign for the Defiance of Unjust Laws in 1952, Mandela was elected the national Volunteer-in-Chief. The Defiance Campaign was conceived as a mass civil disobedience campaign that would snowball from a core of selected volunteers to involve more and more ordinary people, culminating in mass defiance. Fulfilling his responsibility as Volunteer-in-Chief, Mandela traveled the country organizing resistance to discriminatory legislation. Mandela was later charged and brought to trial for his role in the campaign. But the court found that Mandela and his co-accused had consistently advised their followers to adopt a peaceful course of action and to avoid all violence. But for his part in the Defiance Campaign, Mandela was convicted of contravening the Suppression of communism Act and was given a suspended prison sentence. Shortly after the campaign ended, he was also prohibited from attending gatherings and confined to Johannesburg for six months. Mandela continued his campaign and was promoted to become president of the ANC's Transval organization. However the government again acted to stop him. He was arrested along with 155 other ANC members in 1956 and charged with treason and promoting communism. The trial was to last 5 years. The ANC was in the dock and the government was determined to prove that it was a communist and revolutionary organization. It was during this trial, which Mandela dominated with his skill as a speaker and breadth of vision that he emerged as the ANC's most valued leader. He was eventually acquitted, but he knew his freedom was short-lived.