In 1940 he was invited both by Rabbi Yitzchak Hutner to teach in the Orthodox Yeshiva Chaim Berlin, and by the Jewish Theological Seminary of America to serve as professor of Palestinian literature and institutions. Lieberman chose the offer by the Jewish Theological Seminary. Lieberman's decision was motivated by a desire to "train American Jews to make a commitment to study and observe the mitzvot." {Saul Lieberman and the Orthodox} In Chaim Dalfin’s Conversations with the Rebbe (LA: JEC, 1996), pp. 54-63, Prof. Haim Dimitrovsky relates that when he was newly hired at JTS, he asked Rabbi Menachem Mendel Schneersohn of Lubavitch whether he should remain in the Seminary, and the response was "as long as Lieberman is there." In 1949 he was appointed dean, and in 1958 rector, of the Seminary's rabbinical school.
His preoccupation with the Jerusalem Talmud impressed him with the necessity of clarifying the text of the tannaitic sources (rabbis of the first two centuries of the common era), especially that of the Tosefta, on which no commentaries had been composed by the earlier authorities and to whose elucidation only few scholars had devoted themselves in later generations.
He published the four-volume Tosefet Rishonim, a commentary on the entire Tosefta with textual corrections based on manuscripts, early printings, and quotations found in early authorities (currently this work is available in two volumes). He also published Tashlum Tosefta, an introductory chapter to the second edition of M. S. Zuckermandel's Tosefta edition (1937), dealing with quotations from the Tosefta by early authorities that are not found in the text.
Years later, Lieberman returned to the systematic elucidation of the Tosefta. He undertook the publication of the Tosefta text, based on manuscripts and accompanied by brief explanatory notes, and of an extensive commentary called Tosefta ki-Feshutah. The latter combined philological research and historical observations with a discussion of the entire talmudic and rabbinic literature in which the relevant Tosefta text is either commented upon or quoted. Between 1955 and 1973, ten volumes of the new edition were published, representing the text and the commentaries on the entire orders of Zera'im, Mo'ed and Nashim. Furthermore, in 1988, three volumes were published posthumously on the order of Nezikin, including tractates Bava Kama, Bava Metziah, and Bava Basrah. The entire set was republished in the 1990s in thirteen volumes, and again in 2001 in twelve volumes.
In Sifrei Zuta (1968), Lieberman advanced the view that this halakhic Midrash was in all likelihood finally edited by Bar Kappara in Lydda.
His two English volumes, which also appeared in a Hebrew translation, Greek in Jewish Palestine (1942) and Hellenism in Jewish Palestine (1950), illustrate the influence of Hellenistic culture on Jewish Palestine in the first centuries C.E.
Other books of his were Sheki'in (1939), on Jewish legends, customs, and literary sources found in Karaite and Christian polemical writings, and Midreshei Teiman (1940), wherein he showed that the Yemenite Midrashim had preserved exegetical material which had been deliberately omitted by the rabbis. He edited a variant version of the Midrash Rabbah on Deuteronomy (1940, 19652). In his view that version had been current among Sephardi Jewry, while the standard text had been that of Ashkenazi Jewry. In 1947 he published Hilkhot ha-Yerushalmi which he identified as a fragment of a work by Maimonides on the Jerusalem Talmud. Lieberman also edited the hitherto unpublished Tosefta commentary Hasdei David by David Pardo on the order Tohorot. The first part of this work appeared in 1970.
A number of his works have appeared in new and revised editions. Lieberman served as editor in chief of a new critical edition of Maimonides' Mishneh Torah (vol. 1, 1964), and as an editor of the Judaica series of Yale University, where he worked closely with Herbert Danby, the Anglican scholar of the Mishnah. He also edited several scholarly miscellanies.
He contributed numerous studies to scholarly publications as well as notes to books of fellow scholars. In these he dwelt on various aspects of the world of ideas of the rabbis, shed light on events in the talmudic period, and elucidated scores of obscure words and expressions of talmudic and midrashic literature.
He also published a heretofore unknown Midrashic work that he painstakingly pieced together by deriving its text from an anti-Jewish polemic written by Raymond Martini, and various published lectures of Medieval Rabbis. This Midrashic text was lost on account of vigorous church censorship and suppression. Lieberman's work was published while he headed Machon Harry Fishel.
Jacob Neusner, a leading scholar of the history of rabbinic Judaism, criticized the bulk of Lieberman's work as idiosyncratic in that it lacked a valid methodology and was prone to other serious shortcomings (see Sources below). However, ten years earlier, in an article published shortly after his death, Lieberman strongly criticized Neusner's lack of scholarship in the latter's translation of three tractates of the Yerushalmi. Indeed, Neusner has been accused of being biased against Lieberman, presumably due to Lieberman's strong criticism of Neusner's scholarship.
Lieberman had no children.