Michael Carrick's Middlesbrough side leapfrogged Watford and moved up to third in the Championship table by winning the meeting of two play-off hopefuls.
The Teessiders handed on-loan Aston Villa striker Cameron Archer a debut, and he played a big part in the opening goal, eventually scored by Chuba Akpom, his 15th of the season.
With midfielder Dan Barlaser watching from the stands ahead of his imminent move from Rotherham, Boro forged ahead in the 36th minute when Archer's pace took him clear of the Hornets defence.
His touch deserted him and afforded Watford keeper Daniel Bachmann the chance to block his shot, but the ball lobbed into the air and Akpom, despite slicing his attempted volley, was able to follow up and head into the empty net.