Troy Deeney gave the visitors the lead inside the opening five minutes when he was slipped through on goal by Cristian Battocchio, before lashing a fierce strike into the roof of the net past Lee Grant.
Derby came close to a response when Jamie Ward flashed a low ball across the six-yard box, but Johnny Russell could not stretch to meet it at the back post.
Ward was the next to threaten for the hosts as he found space on the edge of the box before curling an effort just wide of the post.
On the hour-mark Derby took lead through Forsyth, who smashed a strike into the corner following a well-worked team move.
Watford's hopes of getting back into the game took a blow eight minutes later as Marco Cassetti was sent off for his second booking of the afternoon.
Despite their numerical disadvantage, the visitors equalised two minutes later when Mathias Ranegie headed home from Ikechi Anya's cross.
In the 81st minute Derby retook the lead as Hendrick picked the ball up 25 yards out from goal, before firing a strike into the bottom corner and out of the reach of Bond.