Born in Southbridge, Massachusetts, Marcy established a legal practice in Troy, New York, after graduating from Brown University.

After leaving the Polk administration, he resumed the practice of law and became a leader of the "Soft" Hunker faction of the New York Democratic Party.

In this role, he resolved a dispute about the status of U.S. immigrants abroad and directed U.S. diplomats to dress in the plain style of an ordinary American rather than the court-dress many had adopted from Europe.

Marcy served in the militia during the War of 1812, first as an ensign in the 155th New York Infantry Regiment, and later advancing through the ranks to first lieutenant and captain.

Samuel Marcy was a United States Navy officer who was killed on board the USS Vincennes during the American Civil War.

[1] Marcy served as United States Secretary of War in the Cabinet of President James K. Polk from 1845 until 1849, when he resumed the practice of law in New York.

[1] Marcy resolved the Koszta Affair (1853), related to detention of an unnaturalized American resident by Austria, gaining his freedom.

[citation needed] In 1854 Marcy had to deal with the complications growing out of the bombardment of Greytown (now San Juan de Nicaragua), by the United States warship Cyane in retaliation for insults offered the American minister by its inhabitants and for their refusal to make restitution for damages to American property.

[4] Upon the 1854 seizure by Spain of the American vessel Black Warrior, on the ground that this vessel had violated the customs regulations of the port of Havana, some propagandists in the United States Congress as well as Pierre Soulé, the American minister in Spain, seemed to prefer war and make possible the seizure of Cuba.

It was largely due to Marcy's influence that war was averted, Spain restored the confiscated cargo, paid restitution, and remitted the captain's fine.

Three American diplomats met to discuss the future of Cuba, but the resulting Ostend Manifesto was quite unexpected, and Marcy promptly disavowed the document.

A diplomatic disagreement with Britain caused Marcy to reject the Declaration of Paris of 1856, which would have set the rules of international maritime law.

[4] Other affairs that demanded Marcy's attention were a Canadian tariff reciprocity treaty, Commodore Matthew C. Perry's negotiations for naval and trade access with Japan, and a British fishery dispute.