During his tenure with that magazine, he became a leading figure within the Pan Africanist Congress of Azania (South Africa). During the 1970s he headed the United Nations Observer Mission of the Pan Africanist Congress (PAC) in New York City and used this position to popularize the PAC particularly among African-Americans. In 1979 Sibeko was partially successful in a leadership coup against Potlako Leballo.
However he failed to get support from the Second Azanian People's Liberation Army, recruited from the 1976 student protest generation and was shot dead during an argument with them at his flat in Oyster Bay, Dar es Salaam, Tanzania on 12 June 1979. David Sibeko's contribution to the liberation struggle in South Africa unfortunately is little known. In actuality his influence was pervasive and, some would argue, on par with that of Nelson Mandela.
Sibeko adroitly used his position as the head of the PAC Observer Mission at the United Nations as a way to unite diverse sectors of the international black community behind the PAC. A sharp thinker, and, above all, an eminently practical, big-hearted man with an easy laugh, he could be seen coaxing support from a U.S. State Department official one minute, gathering support from the communist Chinese the next, and persuading the Soviet representatives to assist the PAC in some small matter or another after that. One could easily find the same ANC officials that would detract him and the PAC during the day, drinking, eating (he would personally cook for his guests), and laughing at his apartment on West End Avenue in New York City that same evening. Had he remained as a highly effective diplomat as a counterbalance to the revolutionary activities of the younger generation the PAC would have continued as a serious rival to the ANC.