Jacob, son of David and Bluma Robinson and the eldest of seven brothers, was born in the little Lithuanian village of Serijai, near Suwalk on the German frontier, then part of the Czarist Empire. Among his distinguished forebears was Rabbi Yom Tov Lipmann Heller, author of the Mishna commentary Tosephot Yom Tov. His father, a well-known scholar, teacher and maskil of Wistyten (Vishtinetz) and a prominent member of the Jewish community, had been its spokesman on several occasions, and had represented it in meetings both with the Czar and with the Kaiser. A sense of Jewish public service was natural in the Robinson household.
In the summer of 1914, as the war-clouds were gathering, Jacob graduated in the Faculty of Law of the University of Warsaw. He went there not so much from choice, but because Czarist anti-Jewish legislation prevented him from studying at the great Russian centres of higher learning. Shortly after the outbreak of the First World War on 1 August 1914 (in the East), he enlisted in the Russian Army under special recruitment plans for university graduates (universanti). He served for about a year, and then was taken prisoner by the Germans after Vilna fell to them, in 1915. He was to remain in captivity in German Prisoner of War camps until the end of the War. There he had a hard time of it, and during a period of about 30 months he was in no less that eight different POW camps, where he established himself as an unofficial leader and spokesman of the Jewish POW's and of the Russian POW's as well, no doubt an early manifestation of his sense of universal humanism which was to show itself on so many occasions later.