His father was Moshe Aaron Kasner, who was the head of a small yeshiva and his mother was Tziril Aptowitzer.
The scholar Hirsch Perez Chajes appointed him as a teacher in the Israelitisch-Theologischen Lehranstalt (Jewish Theological Seminary) he founded.
In 1924 Aptowitzer was invited to an academic position in Jerusalem but he turned down the offer because of his wife's illness.
His edition of the work of Ra'avyah (Eliezer ben Yoel HaLevi) includes a comprehensive scholarly introduction and copious notes.
Under the sponsorship of the Yad Harav Herzog Institute and the Harry Fischel Institute for Talmudic Research, the work was reprinted (3 volumes, not including the Introduction) and supplemented by a fourth volume (dealing with laws of persons) edited by Rabbis Eliyahu Friesman and She'ar Yashuv Cohen.