He was educated at Street Paul"s School, London and Balliol College, Oxford.
In 1910 he was President of the Oxford Union. In 1953 he was made an Officer of the Order of the British Empire. In 1912 Stein received a Call to Bar, at the Inner Temple.
He served in the Army from 1914 to 1920 (Staff-Captain, Palestine Military Administration and subsequently on Political Staff, EEF, in Jerusalem and at General Headquarters in Cairo from 1918 to 1920).
He was Political Secretary of the World Zionist Organisation from 1920 to 1929. He was Honorary Legal Adviser to the Jewish Agency for Palestine from 1929 to 1939.
He was President of the Anglo-Jewish Association from 1939 to 1949. He was President of the Jewish Historical Society of England from 1964 to 1965.
Stein was firstly Liberal candidate for the Dover division of Kent at the 1922 General Election.
The Unionists held the seat. He was then Liberal candidate for the Kensington North division of London at the 1923 General Election. This was a Unionist seat and not a good prospect either as the Liberals had come third in 1922.
However, he did manage to increase the Liberal share of the vote;
He did not contest the 1924 General Election.
He was then Liberal candidate for the Bermondsey West division of London at the 1929 General Election. He might have entertained hopes of regaining the seat, however the Unionists who had not run a candidate in 1923, chose to intervene.
As a result, Labour comfortably held the seat;
He did not stand for parliament again. After the split in the Liberal Party in 1931 he was active in the National Liberals as Vice-Chairman of their London organisation.