In the 1970s, while working for the Johannesburg City Council, he was recruited into Umkhonto we Sizwe (MK), the then armed wing of the ANC. He was part of a unit tasked with recruiting members for military training.
The unit was later instructed to transform its function from recruitment to sabotage. While some members of the unit left the country, he and others remained in the country to establish such machinery. Their unit was also involved in smuggling MK cadres in and out of the country via Swaziland.
On 14 April 1976 Motlanthe was arrested for furthering the aims of the ANC and was kept in detention for 11 months at John Vorster Square in central Johannesburg.
In 1977 he was found guilty on three charges under Terrorism Act and sentenced to an effective 10 years imprisonment on Robben Island.
After his release in 1987, Mr Motlanthe was tasked with strengthening the trade union movement in the country.
To this end, he worked for the National Union of Mineworkers (NUM) as a national office bearer responsible for education. Among other things, he was involved in training workers to form shopsteward committees.
In 1990 when the banning on the ANC and other political organisations was lifted, Mr Motlanthe was tasked with re-establishing ANC structures in the Gauteng Province.
In 1992 he was elected General Secretary of NUM, succeeding Cyril Ramaphosa who had been elected Secretary General of the ANC.
Mr Motlanthe served two five year terms as Secretary General of the ANC from December 1997 to December 2007. As Secretary General he was responsible for the overall operations of the organisation and ensuring that all its structures, including the ANC Womens League and the ANC Youth League, functioned well.
As Secretary General he was also responsible for the organisations international relations programme, which entailed strengthening and consolidating relations with international political bodies and fraternal organisations in foreign countries.
During the 2010 FIFA World Cup, Mr Motlanthe led Governments Inter-Ministerial Committee that oversaw South Africas preparations for the hosting of the 2010 FIFA World Cup.
He was the Deputy President of the African National Congress (ANC) from December 2007 until December 2012.