Chris Porter doubled his side's lead after 77 minutes as the Rams bounced back from back-to-back defeats to turn in a convincing display and do a domestic double over a Watford side now without an away win since October.
The teams may have started the day level on points, but it was the hosts who looked the more assured in the opening stages.
Rob Hulse had a header saved by Scott Loach and Paul Green also tested the Watford goalkeeper before Tonge broke the deadlock after 12 minutes.
The on-loan Stoke midfielder found himself in plenty of space out on the right flank and unleashed a well-struck 30-yard effort that went in via the underside of the crossbar.
Green fired over from the edge of the penalty area 20 minutes later and Danny Graham directed a header over as the Hornets struggled to get a foothold back in the game.
Stephen Bywater easily dealt with a tame 25-yard Jon Harley free-kick as the first half fizzled out.
A Tonge free-kick six minutes after the restart unsettled the Watford defence and Martin Taylor could only clear as far as Green, who shot at at Loach from fully 25 yards.
Bywater then produced a fine instinctive save to keep the Hornets at bay after 54 minutes.
Henri Lansbury was afforded too much space on the left channel and dispatched a rasping 20-yard drive that Bywater did well to tip round the post.
Hulse went close from a similar range at the opposite end of the pitch as the game threatened to burst into life just before the hour mark.
Buoyed by his earlier strike, Tonge tried his luck from 20 yards, but was narrowly off target, as was Harley from 25 yards.
Porter put the game beyond Watford with 13 minutes remaining with his second goal against the Hertfordshire club this season, having netted the only goal when the sides met at Vicarage Road back in December.
Jay DeMerit slipped as he attempted to deal with a punted long ball and Porter pounced, drilling low past Loach from 12 yards for his third goal of the campaign for the Rams.
Green again went close and Bywater also kept out Graham's close-range effort as the Rams bounced back from setbacks against Swansea and West Brom to snatch all three points.