Nicholas Fish joined the American military effort after studying law with John Morin Scott in New York. In 1776 he served as Scott's aide and became a major in a New York regiment. He participated in the Saratoga and Monmouth campaigns and worked as a division inspector under Baron von Steuben, helping with the discipline and organization of Continental troops. His service combined field command with the staff duties needed to sustain the army.

In 1779 Fish accompanied the Sullivan expedition against Haudenosaunee settlements in New York. He subsequently served in the Virginia campaign with Lafayette and took part in the siege of Yorktown in 1781. During the assault on Redoubt 10 he served under Alexander Hamilton in the force attacking the British defenses. He ended the war with the rank of lieutenant colonel and became an original member of the Society of the Cincinnati, later holding leadership responsibilities in its New York society.

After the war Fish became New York's first adjutant general, appointed in 1784. President Washington selected him as a supervisor of federal revenue in 1794. He also served as a New York City alderman and participated in local defense arrangements during the War of 1812. A Federalist, he sought congressional and lieutenant-governorship elections without success. These later activities extended his public life beyond the Revolution into state administration, municipal government, and party politics. He died in New York in 1833.