Born in Russian Empire, he served as rabbi of several Eastern European Jewish communities and escaped to the United States in 1940 following the German occupation of Poland.

He arranged the successful transfer of the entire Mir yeshiva from Lithuania to Shanghai, providing for its support for five years, and obtaining visas and travel fare to bring all 250 students and faculty to America after World War II.

In 1914, when Rakaŭ was flooded with thousands of refugees fleeing Russia with the outbreak of World War I, Kalmanowitz founded a rescue organization that furnished food and clothing.

[8] Later he assisted Rabbi Chaim Ozer Grodzinski in the running of the latter's Vaad Hayeshivos, which provided financial aid and support to European yeshivas and their students.

[11] In 1929, he accepted the position of Rav and av beis din (head of the rabbinical court) of Tykocin (Tiktin), and established a yeshiva in that town.

[12] He was known as a tireless agent for rescue, bombarding government officials with letters and telegrams pleading for help for all Jews trapped in Nazi Europe as well as for those stranded in the Soviet Union.

[9] Even before the U.S. government approved the legality of sending "free currency" (i.e., Swiss francs) into enemy-occupied territory for the purpose of supporting penniless refugees, Kalmanowitz sent money overseas, "ignoring illegalities and FBI threats of arrest".

As Joseph J. Schwartz, chairman of the Joint's European Executive Council, told an interviewer, "There was a rabbi [Kalmanowitz] with a long white beard, who, when he cried, even the State Department listened".

[17] In the years following the war, Kalmanowitz joined Simon Wiesenthal and others in an effort to try and track down Nazi official Adolf Eichman, who was hiding in South America.

[21] In the late 1940s and 1950s, Kalmanowitz inundated government, United Nations, and church officials with appeals to stop persecutions and pogroms against Jews in Egypt and Syria that eventually resulted in the dissolution of those communities.

His efforts led to the passage of a bill according "endangered refugee status" to Jews in Arab lands, paving the way for their immigration to the United States.

Together with the leaders of Jewish communities in Morocco and Tunisia, he helped found the Otzar HaTorah educational network, which established 28 yeshivas and 20 Talmud Torahs, as well as girls schools, in those countries in 1947 and 1948.