Horsfield, who failed to score in his first four appearances for Albion after his £1million move from Wigan, took his tally to four in three games with a goal in each half.
His front line partner, Lee Hughes, put his off the field problems behind him to grab Albion's second.
Hughes, who faces another appearance in court on Friday to answer charges of causing death by dangerous driving and leaving the scene of an accident, struck after 60 minutes with his first goal since November.
Relegation threatened Watford, who have now won just one of their last 10 league games, continue to concede goals at an alarming rate.
They have now leaked 12 in their last three league and cup games and a spectacular 77th minute goal from leading scorer Scott Fitzgerald time did nothing to lift the gloom for them.
To add to their problems strike Heidar Helguson faces a three-match ban after he was sent-off in the last minute for a crude challenge on the Albion right-back Bernt Haas.
Horsfield's first goal fired the game into life after a dismal opening 40 minutes.
Watford opted for the bold approach of playing with three strikers in the form of Bruce Dyer, Helguson and Paul Devlin.
But, apart from a 13th minute cross shot from Helguson that was comfortably held by goalkeeper Russell Hoult, Watford's three pronged attack struggled to pose a threat.
Albion were a little better with their best chances coming from two long-range strikes from Neil Clement.
Predictably it needed a mistake for the deadlock to be broken. Wayne Brown was the unfortunate player as the Watford defender surrendered possession in midfield to Andy Johnson.
Johnson fed a ball into the path of Horsfield who calmly drew Lenny Pidgeley out of the Watford goal before taking the ball around the on-loan Chelsea 'keeper and sliding his shot into an empty net from an acute angle.
Watford threatened briefly to force their way back into the game when Marcus Gayle headed over the bar from close range after 53 minutes.
But their fate was sealed when Hughes and Horsfield struck within the space of six minutes.
Hughes raced on to a pass from Sean Gregan on the hour and weaved his way past Brown before curling his shot from 12 yards beyond Pidgeley.
Horsfield then put the result beyond doubt when he was picked out by a through ball from Thomas Gaardsoe before seeing his right-foot drive from just inside the area bounce into the net off the far post.
Fitzgerald, on the field for just six minutes as a substitute, then earned Watford a consolation strike for an afternoon of toil with a 25-yard drive 13 minutes from time - his ninth goal of the season.