Fish came from both prominence and wealth, his family being of Dutch Americans heritage long-established in New York City. He attended Columbia College, and later passed the bar. Initially working as New York's commissioner of deeds, he ran unsuccessfully for New York State Assembly as a Whig candidate in 1834. After marrying, he returned to politics and was elected to the U.S. House of Representatives in 1843. Fish ran for New York's Lieutenant Governor in 1846, falling to a Democratic Anti-Rent Party contender. When the office was vacated in 1847, Fish ran and was elected to the position. In 1848 he ran and was elected Governor of New York, serving one term. In 1851, he was elected U.S. Senator for the state of New York, serving one term. Fish gained valuable experience serving on the U.S. Senate Committee on Foreign Relations. During the 1850s he became a Republican after the Whig party dissolved. In terms of the slavery issue, Fish was a moderate, having disapproved of the Kansas–Nebraska Act and the expansion of slavery.