[2] He was born as Aaron or Aron Schönberg on December 8, 1813, to a Jewish family in the village of Alzey,[3][4] which was shortly annexed to the Grand Duchy of Hesse after the Napoleonic Wars.

His father, Simon Belmont[a] was the owner of a freehold estate and leading citizen of Alzey, serving as president of the local synagogue for many years.

[6] After his mother's death, he lived with his uncle and grandmother in Frankfurt, where he attended the Philanthropin, a school founded by Mayer Amschel Rothschild, designed to integrate the city's Jewish and Christian communities.

[6] In 1832, his training was rewarded with an appointment as confidential clerk; two years later, he became secretary and traveling companion to one of the firm's partners, which led to his first trip outside Germany to Paris, Naples, and the Vatican City.

[8] In the wake of the Panic of 1837, Belmont was able to use Rothschild credit to buy up wildcat bank notes, securities, commodities, and property at severely depressed rates, sometimes as low as ten cents to the dollar.

Using early modern securitization techniques, he was a pioneer on Wall Street, rapidly shifting money and commodities in complex international spirals of credit New York City had not seen before.

Within three years of his arrival in the city, he had amassed a personal fortune of $100,000 ($2,960,533 in 2023), making him one of the richest men in New York and one of the three most important private bankers in the United States.

In 1847, the United States government granted Belmont & Co. the right to transfer $3 million to Mexico as part of an indemnity paid for land seized in the Mexican-American War.

[12] Belmont was a lifelong member of the Democratic Party who first engaged in political campaigning in 1844, the same year he was naturalized as a citizen, by supporting James K. Polk for president in the hotly contested presidential election.

[16] At the time, New York Democrats were deeply divided into various factions over slavery, with anti-slavery "Barnburners" having bolted in 1848 to support the Free Soil Party candidacy of Martin Van Buren.

Efforts to unite behind Marcy or Stephen A. Douglas at the 1852 Democratic National Convention also failed; Franklin Pierce was nominated as an unexpected dark horse.

[16] Belmont demanded a retraction of at least one Tribune story, but after he was rebuffed by Horace Greeley, he enlisted the Democratic Herald and Evening Post in his defense.

In the letter, Belmont proposed that President-elect Pierce could, through his ministers to London and the Bourbon monarchies in Paris and Naples, create a diplomatic climate favorable to Spanish capitulation.

He reported to Washington that Spain was unstable and desperate for financial relief, but also proposed rebellion in Cuba as an alternative to a direct sale, if blocked by "Castilian pride.

Though Slidell proposed that Belmont participate "on account of the Rothschild influence at Madrid and Paris,"[19] he was not present at their meeting in Ostend, Belgium on October 9, 1854.

[20] After initial resistance from the Dutch foreign ministry, the affair was inflamed in summer 1854 when Gibson, impatient with the State Department's handling of the case, arrived in The Hague personally to pursue his claims, falsely representing himself to Belmont as a special diplomatic agent appointed by Marcy.

Gibson in turn represented himself around Paris as Mason's first secretary, leaking stories to Horace Greeley's New York Tribune which attacked Pierce's foreign policy by suggesting that Belmont utilized his diplomatic post as a banking house and was underwriting the Russian Empire in the Crimean War.

In addition to the Tribune, the Democratic New York Herald (which had turned on the Pierce administration politically, as the result of a patronage dispute) joined in anti-semitic and xenophobic attacks on Belmont for the remainder of his tenure.

Biographer Irving Katz notes that Belmont did not return from Europe until November 1857 and, though he certainly committed money to the Buchanan campaign, no evidence exists as to an exact sum.

[21] Though Belmont hoped to receive a promotion within the diplomatic corps, Buchanan and Lewis Cass, the new Secretary of State, offered him only another four years at The Hague; he declined and resigned his post.

Belmont, who considered Douglas a personal friend and the likely Democratic nominee in 1860,[21] nevertheless publicly endorsed Buchanan's stance in 1858, circulating a petition which urged Congress to admit Kansas into the Union as a slave state and defending the administration against "'Black' Republicans and Know-Nothings" in an Independence Day speech at Tammany Hall.

[21] Belmont's switch from Buchanan to Douglas drew him into the more moderate "Softshell" faction of the New York party, which favored a pluralist, democratic approach on the issue of slavery.

In October 1859, he joined with Samuel J. Tilden and others to organize the Democratic Vigilant Association, a predominantly mercantile group (especially those engaged in trade with the South) to combat "atrocious disunion doctrines," including the abolition of slavery.

[24] In the meantime, Belmont advised Douglas on campaign strategy and gained the candidate's support for a resolution to protect the rights of slaveholders in the territories.

[24] Douglas biographer George F. Milton wrote, "the Committee hoped [Belmont] could smite the Manhattan rock and cause campaign funds to flow."

[26][b] According to one version of events, Belmont also used his influence with European business and political leaders to support the Union cause in the Civil War, trying to dissuade the Rothschilds and other French bankers from lending funds or credit for military purchases to the Confederacy and meeting personally in London with the British prime minister, Lord Palmerston, and members of Emperor Napoleon III's French Imperial Government in Paris.

[29] As early as 1862, Belmont and Samuel Tilden bought stock in the New York World in order to mold it into a major Democratic press organ with the help of Manton M. Marble, its editor-in-chief.

[31] Horatio Seymour's electoral defeat in the 1868 election paled in comparison to the later nomination of Liberal Republican Horace Greeley's disastrous 1872 presidential campaign.

His social companions were largely young rebellious men from well-to-do families; with these connections, he gradually began to introduce European cosmopolitan society to the United States.

[15] She was the daughter of naval officer Matthew Calbraith Perry, captain and commodore in the U.S. Navy, later famous for his expedition to open the trading ports of Japan in 1853.