Janner was president of the Board of Deputies of British Jews, the main representative body of the British Jewish community, from 1978 to 1984, and was a prominent campaigner in the efforts to gain reparations for victims of the Holocaust. In parliament, and outside, he was involved in campaigning for the War Crimes Act 1991, lobbying the Thatcher government to allow legislation to bring those responsible for Nazi atrocities (and now resident in Britain) to justice. He was also a vice-president of the World Jewish Congress until 2009 and of the Jewish Leadership Council until 2015. He was president of the National Council for Soviet Jewry (1979–85) and the Commonwealth Jewish Council.