Noah engaged in trade and law. After moving to Charleston, South Carolina, he dedicated himself to politics.
In 1811, he was appointed by President James Madison as consul at Riga, then part of Imperial Russia, but declined, and, in 1813, was nominated Consul to the Kingdom of Tunis, where he rescued American citizens kept as slaves by Moroccan masters. In 1815, Noah was removed from his position; in the words of US Secretary of State James Monroe, his religion was "an obstacle to the exercise of [his] Consular function." The incident caused outrage among Jews and non-Jews alike.
Noah sent many letters to the White House trying to get an answer as to why they felt his religion should be a justifiable reason for taking the office of consul away. He had done well as consul and had even been able to accommodate the United States request to secure the release of some hostages being held in Algiers. Noah never received a legitimate answer as to why they took the office of Consul away from him. This worried Noah, since he was afraid that this would set a precedent for the United States. He worried that this would block future Jews from holding publicly elected or officially granted offices within the United States.
Noah protested and gained letters from John Adams, Thomas Jefferson, and James Madison supporting church-state separation and tolerance for Jews. Prominent Reform Judaism leader Isaac Harby was moved to write, in a letter to Monroe,
[Jews] are by no means to be considered as a religious sect, tolerated by the government. They constitute a portion of the People. They are, in every respect, woven in and compacted with the citizens of the Republic.
Noah moved to New York, where he founded and edited The National Advocate, The New York Enquirer (later merged into the New York Courier and Enquirer), The Evening Star, and The Sunday Times newspapers.
In 1819, Noah's most successful play, She Would Be a Soldier, was produced. That play has since established Noah as America's first important Jewish American writer. She Would Be a Soldier is now included in college level anthologies.
In 1825, with virtually no support from anyone-not even his fellow Jews- in a precursor to modern Zionism, he tried to found a Jewish "refuge" at Grand Island in the Niagara River, to be called "Ararat," after Mount Ararat, the Biblical resting place of Noah's Ark. He purchased land on Grand Island for $4.38 per acre to build a refuge for Jews of all nations. He had brought with him a cornerstone which read "Ararat, a City of Refuge for the Jews, founded by Mordecai M. Noah in the Month of Tishri, 5586 (September, 1825) and in the Fiftieth Year of American Independence."
Noah also shared the belief among various others that some of the Native American Indians were from the Lost Tribes of Israel, from which he wrote the Discourse on the Evidences of the American Indians being the Descendants of the Lost Tribes of Israel. In his Discourse on the Restoration of the Jews Noah proclaimed his faith that the Jews would return and rebuild their ancient homeland and called on America to take the lead in this endeavor.
On September 2, 1825, soon after arriving in Buffalo from New York, thousands of Christians and a smattering of Jews assembled for a historic event. Noah led a large procession headed by Masons, a New York militia company, and municipal leaders to St. Paul's Episcopal church. Here, there was a brief ceremony- including a singing of the psalms in Hebrew- the cornerstone was laid on the communion table, and the new proclamation establishing the refuge was read. "Proclamation – day ended with music, cannonade and libation. 24 guns, recessional,masons retired to the Eagle Tavern, all with no one ever having set foot on Grand Isle." This was the beginning and the end of Mordecai Noah's venture: he lost heart and returned to New York a couple days later without once having set foot on the island. The cornerstone was taken out of the audience chamber of the church and laid against the back of the building. It is now on permanent display at the Buffalo Historical Society in Buffalo, NY. However, afterwards and despite the failure of his project, he developed the idea of settling the Jews in Palestine and, as such, he can be considered a forerunner of modern Zionism.
From 1827 to 1828, Noah led New York City's Tammany Hall political machine.
In his writings he alternately abhorred and supported southern slavery. He worried that emancipation would irreparably divide the country.
MacArthur Award-winning cartoonist Ben Katchor fictionalized Noah's scheme for Grand Island in his The Jew of New York. Noah is also a minor character in Gore Vidal's 1973 novel Burr.
The modern edition of Noah's writings is The Selected Writings of Mordecai Noah edited by Michael Schuldiner and Daniel Kleinfeld, and published by Greenwood Press.