But the on-loan Stoke midfielder could have missed out on the game, as Derby coach Gary Crosby revealed.
"He has been struggling all week with a virus but he wanted to play for as long as he could," he said. "Michael adds quality, which he showed with the goal and that was with his supposedly weaker foot. He's very comfortable with possession in any area of the pitch and he is a lovely lad as well.
"He's gone to Stoke and they are doing very very well and he's unfortunate not to be involved there more often than he has been, but that's to our benefit at the moment.
"It was pleasing to get a clean sheet as well but I'm pleased for the whole team, not just the back four and the goalkeeper."
For Watford manager Malky Mackay, it was too little, too late as his team were once more left empty-handed on their travels.
They have now taken only three points from their last 11 Championship away games and Mackay admitted: "We are very disappointed because we didn't get started until after half-time.
"Then we showed we could cause problems and create chances, but we have got to start like that.
"The preparation was the same but for some reason we didn't come out of the traps. We've got to make sure we start at the same tempo as the opposition.
"We have to make sure we battle and fight for everything. It was there in patches in the first half but didn't come to the fore until the second when we did get on the front foot.
"We created two golden opportunities in the second half and chances like that have to be taken."
Tonge's long wait for a goal was ended in spectacular style in the 12th minute when a left-footed strike from 25 yards beat Scott Loach via the underside of the bar.
Derby looked capable of adding to that in a first half they controlled but there was more purpose to Watford after the break.
It needed a good save from Stephen Bywater in the 54th minute to keep out a low drive from Henri Lansbury from just outside the area.
The Watford midfielder should have scored in the 70th minute when a mistake by left-back Dean Moxey sold Bywater short, but Lansbury chose to take another touch before shooting and Nicky Hunt got across to put in a decisive tackle.
It proved to be a huge moment in the match because six minutes later, Derby had increased their lead and condemned Watford to another blank away day.
Another defensive error let in Chris Porter and the striker strode clear to drill a right-foot shot into the bottom corner and seal one of the five wins manager Nigel Clough believes will be enough to secure Championship football next season.