Sydney was the son of Jewish immigrants. He was a very bright and principled young man who objected to the war on political grounds (he was a socialist). He spent nearly 3 years in jail doing hard labour for his stand.

Later he became a lawyer and then Labour MP for Nelson and Colne, a seat he held from 1935 until his death in 1968. He was instrumental in passing a Bill to abolish the death penalty. As president of the World Jewish Congress he lobbied strongly for the Americans to bomb train lines running to concentration camps.