Mendes Seixas returned to New York on March 23, 1784, and resumed his former position at the reopened Shearith Israel. He was one of the first ministers to preach a regular Thanksgiving Day sermon. He was also among the fourteen clergymen honored to be chosen to participate in George Washington's 1789 inauguration ceremony as president of the United States. In 1787 Seixas became the first member of his faith to be elected a trustee of Columbia College in New York City and remained such until 1815. He was also one of the College's incorporators. In 1802, he helped found "Hebra Hased Ve Amet", charitable organization. He served on the Board of Regents of what became New York University and as a trustee of the Humane Society of New York.