Mann was born in Frankenthal, Rhenish Palatinate, Germany, the son of a Jewish lawyer.
In 1933, Mann and his wife moved to the United Kingdom, in response to the Nazis' assumption of power.
During the Second World War, Mann became an enemy alien, although he was not interned because the local police force refused to do so.
The same year, he returned to Germany as a member of the Legal Division of the Allied Control Council.
[1] At the same time, he published extensively on international law, in addition to his work as a solicitor.