He studied under Baccio Bandinelli and Jacopo Sansovino (assisting on the design of the Library of St. Mark's, the Biblioteca Marciana, Venice) and closely imitated the style of Michelangelo.
He labored during 1558–1570, in the refurbishment and enlargement of Pitti Palace, creating the courtyard consisting of three wings with rusticated facades, and one lower portico leading to the amphitheatre in the Boboli Gardens.
Ammannati designed what is considered a prototypic Mannerist sculptural ensemble in the Fountain of Neptune (Fontana del Nettuno), prominently located in the Piazza della Signoria in the center of Florence.
The ungainly sea god was placed at the corner of the Palazzo Vecchio within sight of Michelangelo's David statue, and the then 87-year-old sculptor is said to have scoffed at Ammannati— saying that he had ruined a beautiful piece of marble— with the ditty: "Ammannati, Ammanato, che bel marmo hai rovinato!
"[2] Ammannati continued work on this fountain for a decade, adding around the perimeter a cornucopia of demigod figures: bronze reclining river gods, laughing satyrs and marble sea horses emerging from the water.