From Vilna Feinstein joined the Mir Yeshiva in exile in Japan, while helping other refugees escape the horrors of the Holocaust. Eventually, he arrived in the United States in 1941 with Aharon Kotler. While the latter opened a yeshiva in Lakewood, New Jersey (Beth Medrash Govoha), Feinstein served as "mashgiach" for Joseph B. Soloveitchik in Boston. Less than a year later, his uncle, Moshe Feinstein, summoned him to serve at his side as the head of Mesivtha Tifereth Jerusalem in the Lower East Side, Manhattan. During this time, he gained fame for his Talmudic lectures. He was appointed a member of the Agudas HaRabbonim and assisted the Vaad Hatzolah in rescuing Jews and aiding the war refugees in Europe.