Long-serving Spanish midfielder David Silva opened the scoring with a clinical finish after a header by Sterling before the latter doubled thelead.
A sublime pass by Portuguese midfielder Bernardo Silva allowed Brazilian forward Gabriel Jesus, starting his first game since 6 April inplace of Sergio Aguero, to tee up Sterling to tap home.
Substitute Kevin de Bruyne added the third after a beautifully worked move before setting up Jesus to make it
4-0.
Sterling fired into the top corner following Bernardo Silva's pass before completing his hat-trick in the closing stages from close range.
Watford had started brightly but Ederson's fine save to keep out Roberto Pereyra when the game was goalless proved critical.
The Hornets wanted for a penalty when Abdoulaye Doucoure's shot hit Vincent Kompany but referee Kevin Friend waved their appeals away after a video assistant referee review.
City's class shone through as Pep Guardiola's side became the first men's team to win English domestic treble of league, FA Cup and Carabao Cup in the same season.
It was City's sixth FA Cup triumph and their first under Guardiola, who has now won six trophies since taking over at Etihad Stadium in 2016.
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