Former Ryman League striker Scott Fitzgerald, who was playing for Northwood a year ago, clinched the points for Watford with his fourth of the season before missing two further chances to make the points safe.
Lewington's men who had been booed off the field at half time on Tuesday were afforded a standing ovation when they came off at the interval.
"We played superbly in the first half but I was worried that we had nothing to show for it," said the Hornets boss.
"I was afraid after failing to make the most of some really good approach work that we might be hit by a sucker punch with Bradford going up the other end and scoring a soft goal."
In fact, Watford only had one moment of anxiety and that was when Paul Evans sent a 85th minute free-kick over the home wall but the ball curled just wide.
Bradford boss Nicky Law was under no illusion, commenting: "The players worked really hard and their attitude was first class but a lot of their effort was channelled in the wrong direction.
"We were second to the ball and second in all departments from the first minute to the last. The best we could hope for with that performance was to hang on to a point but really were lucky to lose by only one goal."
Law named Watford's Paul Devlin as a "real menace" as the Hornets tore into Bradford in the first half. Devlin had six attempts at finding the net at the end of impressive runs but all went off target.
The same happened to Marcus Gayle who had perhaps the best chance of the half when he found himself in space some 10 yards out from the far post and elected to send a side foot attempt goalwards and only succeeded in clipping the outside edge of the post.
Forward Danny Webber was gifted a chance when he intercepted a mis-hit Nicky Summerbee back pass, but as he broke clear he elected for an early shot and fired straight at keeper Mark Paston.
Watford finally broke through on the hour when Paul Devlin linked up with Micah Hyde who in turn set up Fitzgerald who planted the ball home from six yards.