Born in Germany, Manuel Josephson came to America when he was in his twenties, settling in New York City, where and worked as a merchant. During the French and Indian War, he was a sutler, traveling with the British Army and selling provisions.

He became very involved with Congregation Shearith Israel, where he served as a parnas, in 1762, and as a judge on the congregation’s beit din (religious court), settling religious disputes and matters of interpretation. Josephson was known to have one of the most extensive Jewish libraries in colonial America, and he regularly corresponded with rabbinic authorities in England and Holland on religious questions. He was also known for his sometimes-caustic manner, and one fight with fellow congregants, the 84-year-old Joseph Simson and his sons, ended up in civil court.

In 1759 he married Rachel Judah, daughter of Baruch and Sarah Helbert Judah; they are not known to have had any children. With the outbreak of wat, and the British occupation of New York, in 1776, the couple—like many in the community—fled to Philadelphia. There, Josephson set up a store at 144 High Street and became a leading member of Mikveh Israel. In 1784 he successfully petitioned the congregation to build a mikvah and the following year he was elected parnas, a position he held until 1791.

With the inauguration of George Washington, in 1789, the heads of six synagogues—in New York, Philadelphia, Charleston, Richmond, Newport, and Savanna—agreed to send a joint letter of congratulations to the new president. A year and half of infighting followed, with disputes over who should author and who got to sign this epistle. The congregations of Savanna and Newport ended up presenting separate letters, and in December 1790, Josephson presented a letter to Washington from the four remaining congregations. “The affection of such a people is a treasure beyond the reach of calculation,” Washington wrote in his response.