In 1907, de Sola Pool was invited to become the assistant rabbi of Congregation Shearith Israel — often called the Spanish and Portuguese Synagogue — located in New York City, the oldest Jewish congregation in the United States. He served as its assistant rabbi, under H. Pereira Mendes, until 1919. He left to do work in Israel, but returned to become senior rabbi in 1921, when the synagogue was unable to find a replacement for Mendes after his 1920 retirement. In his detailed history of the congregation, Dr. Pool highlighted with satisfaction its noteworthy diversity: “Probably in no other congregation in the world is there so wide an ingathering of Jews from the four corners of the earth as is to be found in Shearith Israel...Throughout the congregation’s three centuries, its history has been marked by the integration of the elements which have come to it from the sixty or more countries of the earth. Within the compass of the congregation the motto of the country, E pluribus unum, has happy fulfillment.”