True, the midfielder's 88th minute headed equaliser only earned a point, but that goal moved the Addicks another small step closer to safety, and if they continue to show the same level of character that enabled them to turn round what looked to be a lost cause at half-time, then The Valley may yet still be hosting Premiership football next season.
But it was substitutions, just as much as the never-say-die attitude of Alan Pardew's men that transformed this game. Ambrose's goal stemmed from the right boot of replacement Dennis Rommedahl, who had only been on the pitch a matter of seconds when he sent over his precision cross.
There was still time for the Dane to almost set up a dramatic winner for another replacement, Kevin Lisbie, but he had two bites at the cherry and failed to grab the glory.
But if Rommedahl's introduction was crucial, so was a decision made by Aidy Boothroyd when he chose to introduce Johan Cavalli for the excellent Damien Francis midway through the second half.
Francis had been deployed behind lone frontman Tamas Priskin in a new-look, attack-minded 4-2-3-1 formation and the result was that Watford played with an attacking swagger in the opening 45 minutes, that the Addicks simply couldn't handle.
Charlton predictably offered more of an attacking threat in the second half, but the Hornets were hardly under the cosh when Boothroyd made that substitution and chose to revert to a 4-4-2 shape.
Within minutes, Luke Young had latched on to a delightful Alexandre Song pass to give Charlton hope, but the Hornets still looked like they would hold on until Ambrose struck.
The consolation for Watford was that the point was enough to lift them off the Premiership basement for the first time in three months, but that matters little when you are ten points adrift of safety with only nine games remaining.
Watford took the lead after 15 minutes when a lovely flick from Tommy Smith found Priskin, who did well to get round Osei Sankofa before standing up a superb cross from the right byline.
Francis connected well enough with his header, only to be denied by an excellent Scott Carson stop, but Hameur Bouazza, coming in at the far post, kept his head and rifled the rebound into the roof of the net from six yards.
If that wasn't good enough, the Hornets doubled their lead with another incisive move six minutes later.
Adrian Mariappa started it with a ball forward, Francis flicked on to Smith in a central area and the winger weighed up his options before attacking the left side and striking an angled left-footed delivery that Priskin just failed to make contact with, but Francis arrived behind him at the back post to apply the killer touch.
Charlton though, gave themselves hope midway through the second half when Song threaded a left-footed pass down the inside right channel and the overlapping Young got past Mariappa and calmly side-footed the ball across Richard Lee and inside the far corner.
And the comeback was complete two minutes from time when Song spread the ball out to Rommedahl on the right and the Dane flighted over a deep cross which Ambrose headed down and inside Lee's right-hand post.
But there was still time for Charlton to so nearly win it deep in injury-time when they broke on the counter-attack and had a three-on-two advantage. As Rommedahl approached the area, he picked out Lisbie to his left in space.
The substitute went round Lloyd Doyley before firing in a shot that Lee did well to block, but the rebound fell for Rommedahl who again found Lisbie, but this time the striker fired well wide.