The match was an end-to-end affair but it looked to be heading for a draw until Healy's intervention.
Preston opened impressively forcing four corners in the opening two minutes and captain Chris Lucketti had a header tipped over by Watford's on loan keeper Lenny Pidgeley.
Lucketti then had a header tipped round the left-hand post from a Graham Alexander corner.
Watford's first chance came in the 13th minute when Wayne Brown's cross found Scott Fitzgerald whose header was saved by Jonathan Gould.
In the 17th minute Eddie Lewis had a shot saved by the Watford keeper while at the other end Paul Devlin brought a great save out of Gould in the 25th minute.
Preston opened the scoring in the 50th minute when Healy found Paul McKenna. He set up Pawel Abbot who fired a 15-yard shot past Pidgeley.
Watford equalised in the 65th minute when a Fitzgerald cross from the right fell to Danny Webber and he scored from six yards with a right-foot shot.
Preston snatched all three points with a goal in the last minute when Alexander and Abbott combined to set up Healy who fired home the winner.