Born in Prague, Weltsch studied Law and Philosophy at the Charles University. He lived and worked in Prague until 15 March, 1939, and left the city with Max Brod and his family on the last train out of Czechoslovakia. In Palestine, he worked as a librarian in Jerusalem until his death in 1964.
He had one daughter, Ruth Weltsch (1920-1991), with his wife Irma Herz (1892-1969). They married in August 1914. The publisher, journalist and important Zionist Robert Weltsch was Felix Weltsch's cousin. He published the Jewish paper Juedische Rundschau in Berlin, in which he criticized Adolf Hitler's anti-Semitic politics at the beginning of the 1930s.
Weltsch's works around deal with the subjects of Ethics, Politics and Philosophy. For his ethical and political publications Weltsch received the Ruppin- Prize from the city of Haifa in 1952. His most important work was the Jewish-Zionist weekly paper Selbstwehr (self-defense), which he led from 1919 until 1938. With this work and hundreds of articles he became one of the most important personalities in Jewish life next to Martin Buber, Chaim Weizmann and Hugo Bergmann, his early school friend.
Weltsch wrote remarkable essays on philosophers like Henri Bergson and Christian von Ehrenfels, who was the most influential teacher for Weltsch. This was in so far unusual, as most of Weltsch's colleagues and student friends were more following the ideas of Franz Brentano.