From 1909 to the beginning of the Second World War, Cohen directed the English department of the Zionist Central Office in Cologne and later in Berlin.
During the years 1918–1921 he carried out a number of important diplomatic and fund-raising missions on behalf of the Zionist leadership.
These took him to Poland and Hungary, where he investigated and reported on the pogroms and other anti-Jewish acts of violence; and to Jewish communities in Australia, Hong Kong, India, China, and Japan.
He was also a member of the Board of Deputies of British Jews, and in 1946 he was appointed head of its Foreign Affairs Committee delegation to the peace conference in Paris.
When working in Germany, Cohen became the Berlin correspondent for The Times and The Manchester Guardian, and continued to represent the latter at every Zionist Congress up until 1946.