He was born in Cardiff, and trained as a lawyer, attending Trinity Hall, Cambridge, and Harvard Law School. He became a barrister and was appointed a QC (Queens Counsel) in 1971.
He represented Leicester North West then Leicester West in the House of Commons from 1970 until his retirement in 1997. His predecessor in the seat was his father, Sir Barnett Janner. It is now held by Patricia Hewitt.
He was president of the Board of Deputies of British Jews from 1978 to 1984, and was a key campaigner in the efforts to get reparations for victims of the Holocaust.
He was created Lord Janner of Braunstone in 1997. He is a member of the All-Party Parliamentary Group against Anti-Semitism, and the Israel APPG.