He was born in Venice, the son of Paola Foscari and Zaccaria Sagredo. Nicolò's career was initially hampered by his father's reputation as a coward. In May 1630, at the Battle of Valeggio, a part of the War of the Mantuan Succession, the elder Sagredo had deserted in the midst of a battle that saw Venetian forces thoroughly trounced. His family disgraced, Nicolò was unable to embark on the life of politics he might otherwise have begun at this time.