He studied at German universities and the Hochschule für die Wissenschaft des Judentums in Berlin.
He was a member of the Democratic Fraction, an opposition group at the Zionist Congress that lobbied for cultural as opposed to political Zionism.
In 1928 he founded the Judisk Tidskrift, was engaged as a translator as well as scientific writer for different encyclopedias, since 1935 he became a professor at Stockholm University.
Ehrenpreis was the chairman of Arbetsutskottet för hjälp åt Polens judar, devoted to sending aid in the form of food, medicine, clothes and money, primarily to Poland but increasingly also to other parts of Nazi-occupied Europe.
Ehrenpreis was also the Chairman of the Swedish Section of World Jewish Congress from its institution in 1944 and was succeeded by historian and pioneering scholar of antisemitism, Hugo Valentin.