Adler was born in Arkansas, a graduate of University of Pennsylvania in 1883 and gained a Ph.D. from Johns Hopkins University in 1887, where taught Semitic languages from 1884 to 1893. He was employed by the Smithsonian Institution for a number of years, with a focus on archaelogy and Semitics, serving as the Librarian from 1892-1905. He was a founder of the Jewish Welfare Board, and an editor of the Jewish Encyclopedia, and part of the committee that translated the Jewish Publication Society version of the Hebrew Bible published in 1917. At the end of World War I, he participarted in the Paris Peace Conference in 1919.
He was president of the Dropsie College for Hebrew and Cognate Learning 1908 to 1940 and Chancellor of the Jewish Theological Seminary of America. He was also a contributor to the New International Encyclopedia.
Adler was a bachelor much his life, marrying Racie Friedenwald of Baltimore in 1905, when he was 42. They had one child, a daughter Sarah.
His papers are held by the Center for Advanced Judaic Studies at the University of Pennsylvania.