Reverend Dr. Jacob Mayer was a European-born American rabbi who served congregations in the Reform Judaism movement in the late 19th century. He obtained the pulpit in 1874 at Har Sinai Congregation in Baltimore, founded in 1842 and the longest continually functioning congregation adhering to Reform since its inception. Mayer was forced to leave two years later amid charges that he had been an apostate who had converted to Christianity and had been a missionary in Africa, though he vigorously denied the charges.