Nadler was initially ordained as an Orthodox rabbi by Rabbi Aryeh Leib Baron (1911–2011) of Yeshiva Merkaz ha-Talmud in Montreal, and received a second ordination from Rabbi Zvi Hirsch Tennenbaum of the Orthodox Rabbinical Court of Justice in Boston. He studied Talmud and Rabbinical Codes for two years in the Rabbinical Program at Jews' College in London, England, and also studied privately for many years with Montreal's Chief Rabbi Pinchas Hirschsprung. During the period 1984 - 1992, Nadler became Rabbi of Congregation Shaar Hashomayim[disambiguation needed] in Montreal, Canada's oldest and largest traditional Jewish congregation. Nadler was the first Orthodox-ordained rabbi at this formerly Conservative congregation, which left the Conservative Movement during his tenure. After resigning from that congregation, he served as Director of Research of the YIVO Institute in New York City (1992–1999), which includes the world's largest Yiddish Archives and Library.
While at YIVO, Nadler led international Jewish efforts to repatriate libraries, archives and Torah scrolls in Lithuania that had been plundered and confiscated by the Nazis, and later held by Soviet authorities. His direct negotiations with then-President of Lithuania, Algirdas Brazauskas, led to the release, to the New York offices of YIVO for reproduction and cataloguing, of archives that had belonged to YIVO in pre-war Vilna (today, Vilnius, Lithuania), after extensive international coverage of the story.
Nadler has been a public critic of the Chabad-Lubavitch movement, and of the late Lubavitcher Rebbe, Menachem Mendel Schneerson. His criticisms of Schneerson in The New York Times, and his subsequent negative assessment of Lubavitch Messianism in a series of articles for The New Republic, were denounced by many Orthodox Canadian and American Rabbis. This was one of many factors that ultimately led Nadler to leave the Orthodox Rabbinate. His evolving theological and historical perspectives, evident in his writings, were however the major reason for his having turned away from Orthodox Judaism. On an extended sabbatical/leave of absence from Drew University since 2011, Nadler is currently the Rabbi of Congregation Beth El in Montreal, an egalitarian conservative congregation, and a visiting professor of Jewish Studies at McGill.
Prior to his appointment at Shaar Hashomayim, Nadler had been the Rabbi of The Charles River Park Synagogues in Boston, at the time an "Orthodox" congregation, with a mixed seating section, and was a member of the Boston Vaad HaRabonim, serving as a Dayan, or judge, on its Rabbinical Court from 1980-1982. While at YIVO he also served as Rabbi of the Fort Tryon Jewish Center in New York City, an independent synagogue. Nadler has been a frequent critic of Haredi rabbis and institutions, including the Dean of one of America's most prominent Yeshivas (Rabbinical schools), Lakewood, New Jersey's Beth Medrash Govoha, Rabbi Aryeh Malkiel Kotler, whom he accused, in The Forward, of approving a racist, anti-Gentile book, "Sefer Romemut Yisrael," written by one of the Yeshiva's students. At the same time, Nadler has published scholarly studies of some of the major sects of Hasidism, such as Satmar, Munkatch and Slonim, in addition to a widely noted analysis of the culinary habits of the Hasidim on the Shabbat and Jewish holidays ("Holy Kugel") that has been widely mistaken for an anti-Hasidic satire. In 2007, however, Nadler published two articles that strongly defended the Orthodox Jewish community: one in response to Noah Feldman's negative expose of Modern Orthodoxy in the New York Times magazine, and the other—published in the Montreal English daily newspaper, The Gazette, in defense of that city's large Hasidic community that has been frequently criticized for uncivil behavior towards their French neighbors.
Nadler's book, The Faith of the Mithnagdim: Rabbinic Responses to Hasidic Rapture, which developed out of his Ph.D. thesis, "A Religion of Limits: The Religious Thought of Rabbi Pinchas of Polotsk" from Harvard University under the late Isadore Twersky, is an investigation of the theology of the rabbis who opposed the Hasidic movement in late 18th-early 19th century Eastern Europe. Nadler is a regular book critic for The Forward, an American national Jewish newspaper, to which he has contributed more than one hundred articles, essays and book reviews. He was a prolific feature-article writer for Jewish Ideas Daily (now called Mosaic), penning more than fifty articles and reviews, and regularly writes for the Jewish Review of Books. Nadler is widely considered a leading, and provocative public intellectual in contemporary Jewish thought.
- New York Times news about Allan Nadler
- Nadler, Allan, "The Faith of the Mithnagdim: Rabbinic Responses to Hasidic Rapture" (Baltimore: The Johns Hopkins University Press, 1997)