The Hornets, who sealed automatic promotion back to the Premier League at the first attempt, saw off a late Villa comeback to hold on to all three points.
Xisco Munoz’s side took the lead on 10 minutes through forward Emmanuel Dennis on his debut.
Ismaila Sarr doubled the hosts’ lead three minutes before half-time, beating goalkeeper Emiliano Martinez with a cruel deflection off Villa captain Tyrone Mings.
There was another debut goal for Watford substitute Cucho Hernandez, who added a third from the edge of the box seconds after coming on to the pitch.
Against the run of play, Aston Villa found the back of their net three minutes later with a brilliant half-volley from John McGinn.
Danny Ings then scored his first Villa goal from the penalty spot in injury time after Bertrand Traore was tripped by Adam Masina, but it was too little too late for Dean Smith’s side.