In 1927 he left Europe to become one of the first Hasidic Rebbes in America, establishing his court on the Lower East Side of New York City and attracting many American Jewish youth with his charismatic and warm personality.

He also played a role in American Jewish leadership with positions on Agudath Israel of America, the Moetzes Gedolei HaTorah, and Holocaust rescue organizations.

[4] At the beginning of World War I, the Russian army entered the town of Boiany (Boyan) and destroyed the Jewish neighborhood.

[7] At that point he entertained an offer to head the Hasidic community of Drohobych in Western Ukraine, and another to lead an organized chaburah (group) of Boyaner Hasidim on New York's Lower East Side.

Rabbi Mordechai Shlomo acquiesced and embarked on an 11-month pilot trip beginning in December 1925, visiting several New York neighborhoods as well as Boston, Chicago and Philadelphia.

[10] On Hanukkah of that year, the Boyaner Hasidim called a meeting to raise money for the purchase of the building at 247 East Broadway as a kloiz.

[21] In conjunction with Agudath HaRabbanim, he helped organize the Vaad Hatzalah during World War II and assisted the rescue of Torah leaders in Poland and Eastern Europe.

[21][22] After the war, he was elected president of Vaad HaEzra, in which capacity he raised funds to help Holocaust survivors in post-war Europe.

[24] Like his father and grandfather before him, the Rebbe served as president of the Kollel Vohlin (Association of Ukrainian Jews in Palestine), distributing large amounts of charity money in Israel each year.

[25] He also gave all the pidyonos (redemption-money) he received from people giving him kvitlach to the Boyaner chesed fund, earmarked for the poor in Israel.

On his first trip, he visited his brother Rabbi Yisroel, the Boyaner Rebbe of Leipzig and Tel Aviv, whom he hadn't seen in 22 years.

On his trip to Israel in 1953, the Boyaner Rebbe of New York laid the foundations for a new Ruzhiner Torah centre in the New City of Jerusalem.

[32] Tens of thousands accompanied his bier in a funeral procession in the streets of Jerusalem to the Mount of Olives, where he was buried at night on the heights of the mountain.

The Hasidim then asked the Rebbe's daughter Malka and her husband, Rabbi Dr. Menachem Mendel Brayer, a lecturer at Yeshiva University, to offer one of their two young sons for the leadership.

The lot fell to the younger son, Nachum Dov (born 1959[33]), who then enrolled at the Ruzhiner yeshiva in Jerusalem to prepare himself for the task.