Darius Henderson's goal at the end of the first half secured the points, but it came soon after referee Andy Penn had dismissed Republic of Ireland Under-21 international John-Joe O'Toole for supposedly denying Stephen Clemence a goalscoring opportunity.
There was no doubt that the Hornets midfielder pulled back Clemence, but it happened outside the penalty area and there were Watford defenders with more pace than the Leicester skipper comfortably able to get back and cover.
To add to the fury of the home side, the referee didn't initially see the instant, but chose to show red on the recommendation of assistant Gary Young.
It was undoubtedly a harsh decision, but the resolve of character of Aidy Boothroyd's men ensured justice was done against an abject Foxes outfit.
Watford's brand of direct, physical football may not be to everyone's liking, but there is no doubting its effectiveness, particularly when the perpetrators are able to build up a head of steam.
The Hornets did just that for a 15-minute period in the first half but somehow Leicester survived.
Keeper Ben Alnwick did well to thwart Henderson twice and also O'Toole, while the lively Jobi McAnuff was among others who had chances during this period.
It wasn't quite all one-way traffic during this spell as an assistant's flag denied Leicester a goal after Dudley Campbell had bundled Steve Howard's lob over the line following an exquisite Matt Oakley pass.
However, having seen their opponents reduced to ten men, the Foxes would have at least expected to go into the break all-square - but it wasn't to be.
A deep free-kick from the left from full-back Mat Sadler was headed back into the danger zone by Leigh Bromby and Henderson found himself in space to prod home his 11th goal of the campaign.
Watford were content to rely on their organisation and resolve after that, and that was more than enough to deal with a Leicester side who mustered just two attempts on target all match.