The Gulls had by far the greater share of possession throughout the game, but due to some wayward shooting were unable to give Hornets' keeper Michael Hunter too many problems.
In contrast, Horsham posed a constant threat on the counter attack, and Heale's goal in injury time will have been of much relief to John Batch as his side continue to sit just below the playoff places.
The game started at a fast pace, with Alex Rhodes having a shot turned behind for a corner as early as the 2nd minute.
Up the other end, Tony Nwachucwu did well to get in behind Ashley Dumas and drive into the area, although his shot went wide of the target with 8 minutes on the clock.
Canvey had two chances in quick succession after 15 minutes, with Alex Rhodes seeing his dipping volley going just over the crossbar.
Soon afterwards Jay Curran was fouled just outside the penalty area, and Michael Hunter did well to dive to his left and turn Rob King's resulting free kick behind for a corner.
The visitors remained a threat, and will wonder how they didn't take the lead after half an hour. A goalmouth scramble in the Canvey area caused much panic, with James Russell and his defence doing well on two occasions to prevent the ball hitting the back of the net.
Just before half time, Horsham's Senny Diedhiou was extremely lucky not to be given his marching orders following a dangerous two-footed tackle on Alex Rhodes. However, the referee Mr Evetts showed the Hornet's striker a yellow card instead of the anticipated red, and Diedhiou remained on the field.
With the scores level at half time, Canvey had the first chance after the restart.
Recent acquisition Danny Spendlove slid a tremendous through ball into the path of Leon Gordon on 54 minutes, but the Canvey stalwart saw his shot saved from a tight angle.
Two minutes later and the visitors should once again have opened the scoring. A cross field ball was misjudged by John Easterford, who's diving header failed to connect and allowed Ray Freeman through on goal. However, finding himself one-on-one with James Russell, the Canvey keeper did brilliantly to stand up until the last minute before saving the shot with his legs.
Leon Gordon was replaced by Jason Hallett on 66 minutes, with Horsham bringing Marvin Roberts into the game in place of Kery Kedze soon afterwards.
Canvey were starting to camp in the Horsham half, but despite the amount of possession were not able to work Michael Hunter.
Jay Curran had a great chance to do this on 84 minutes when he connected with Alex Rhodes left wing cross at the back post, but his volley went tamely over the crossbar.
Alex Rhodes then had a shot of his own 2 minutes later, with a stinging left footed drive from distance being well saved.
The referee signalled for 3 minutes to be added on at the end of the regulation 90, and the match seemed to be heading for a draw.
However, when Canvey earned a free kick on the right wing, Michael Alaile's pinpoint ball into the area found Danny Heale at the back post who controlled the ball before blasting it into the net from close range.
Horsham forced a corner as they pushed for a dramatic equaliser, but Canvey held on to claim what was an unlikely yet deserved 3 points.