In 1852, Belmont acted as James Buchanan’s presidential campaign manager in the state of New York, and then donated liberally to the campaign of Franklin Pierce, who had defeated Buchanan for the Democratic nomination.  After Pierce’s election, the new president rewarded Belmont with appointment as the U.S. minister to the Netherlands.  During his term (1853-1857), Belmont negotiated trade and extradition treaties between the two countries and helped draft the Ostend Manifesto (1854) that called for the annexation of Cuba by the United States.  He again supported Buchanan’s presidential aspirations in 1856.  When the President Buchanan ignored Belmont’s petition in 1857 to become U.S. minister to Spain, Belmont resigned as the Dutch minister, returned to New York City, and threw his support to Buchanan’s Democratic rival, Senator Stephen Douglas. 

When the Democratic Party split in 1860, Douglas chose Belmont to manage his presidential campaign.  The next year, he was selected to chair the Democratic National Committee, a position he would hold for the next eleven years.  During the Civil War, he sided with the War Democrats in favor of the Union cause and influenced European financiers, like the Rothschilds, to withhold financial assistance from the Confederacy.  In 1862, Belmont joined with other prominent Democrats to purchase the New York World, and named Manton Marble, one of his best friends, as the newspaper’s editor.  They transformed the publication into the leading voice of Democratic opinion until Marble retired in 1876.  In 1864, Belmont helped secure the Democratic presidential nomination for General George McClellan, and two years later resisted President Andrew Johnson’s efforts to merge his National Union Party with the Democratic Party.

In 1868, Belmont’s candidate for the Democratic presidential nomination, Chief Justice Salmon Chase, lost to Horatio Seymour because Chase’s support of voting rights for black men alienated him from the Democratic mainstream.  As Belmont foretold, General Ulysses S. Grant trampled Seymour in the general election.  In late 1871, Belmont joined other prominent New York City Democrats to cooperate with efforts to topple Boss Tweed and his cohorts, whose corruption had finally resulted in indictments.  After Tweed was forced to resign from Tammany Hall in January 1872, Belmont was among the reform Democrats elected to its council.  Later that year, Belmont resigned as chairman of the Democratic National Committee.  Thereafter, he became less active in politics, although he did work unsuccessfully for the Democratic nomination of Senator Thomas Bayard of Delaware in the next three presidential campaigns.

Belmont continued to be active in social affairs and financial endeavors, becoming one of the richest men in America.  He helped popularize horseracing in the United States, establishing the Belmont Stakes race in 1867.  Known as a gourmand, he hosted lavish dinner parties.  He was an avid art collector and served as the first board president of the Academy of Music (1878-1884).  He died in New York City on November 24, 1890.