By the 1760's there were about 300 Jews in New York. Shearith Israel appointed the 23 year old Gershon Mendez Seixas as its Hazzan because like all congregations in the colonies, there were no ordained Rabbis. In 1775 Seixas who was an ardent patriot convinced his divided congregation to close down rather than remain in occupied New York. He packed up and took his family to his father-in-law's house in Connecticut. In 1780 he relocated to Philadelphia and became the Hazzan of Congregation Mikveh Israel. He prayed for the patriots and zealously defended religious liberty, in his 1783 address at the Commonwealth of Pennsylvania's Council of Censors in December, opposing the adoption of a religious test for prospective office holders. He returned to Shearith Israel. Four years later he became the first Jew to be elected a trustee of King's College, the precurser to Columbia University and remained such until 1815. He was also one of the College's incorporators. In 1789 Seixas was one of three clergymen who participated in the Inauguration of George Washington. Heeding President Washington's declaration of Thursday, November 26, 1789 as the first official Thanksgiving, Seixas led his own congregation in a service marking the occasion, delivering a sermon.
1780- Patriots who had fled to Philadelphia wanted to establish the city's first permanent congregation and selected Mendes Seixas to officiate, who established the historic Congregation Mikveh Israel.