Watford travelled to Millwall in the thick of a fruitful spell in front of goal, and Lewis McGugan put them ahead inside the opening 20 minutes, but Steve Morison - introduced by Lions boss Ian Holloway at half-time - drew the hosts level midway through the second half and appeared to have salvaged a point.
Almen Abdi then thought he had had the final say when he restored the Hornets' lead four minutes from time, but Woolford struck in the fifth added minute to halt Watford's play-off push.
With both teams needing a win for their respective end-of-season ambitions, the game began at a frantic pace as Carlos Edwards fired over from 20 yards early on.
The first big chance fell to Gabriele Angella, but the Watford centre-half could only glance McGugan's inviting corner just wide of the far post from six yards.
Watford's in-form duo of McGugan and Troy Deeney were linking up well and the pair combined to give the visitors the lead on 17 minutes.
McGugan's dummy allowed Deeney to back-heel the ball back into the midfielder's path before driving home past David Forde from 20 yards for his 10th goal of the season.
Watford could have doubled their lead moments later but Ford was out quickly to block Deeney's close-range effort on the angle.
That shocked Millwall into life and the hosts should have levelled before the break, but Owen Garvan could only shoot straight at Manuel Almunia from Stefan Maierhofer's low cross.
The hosts began the second half on top with Maierhofer heading over from six yards on 55 minutes after meeting Lee Martin's outswinging free-kick.
And Millwall finally got their reward shortly after when second-half substitute Morison volleyed home from the angle following Woolford's corner.
As the hosts pushed for the winner the game became stretched, and Fernando Forestieri should have reclaimed the lead for the visitors on the break but his low drive struck the legs of Forde.
Having brought three strikers on, Holloway was desperate for the three points and Nicky Bailey was inches from a spectacular 25-yard winner but was denied by the fingertips of Almunia.
And that save proved vital as, just a minute later, Abdi thought he had stolen the points for Watford as his deflected 18-yard effort found the corner of Forde's net with just four minutes remaining.
But Millwall kept coming and Woolford snatched a dramatic last-minute equaliser, squeezing his left-footed effort through Almunia, to earn his side a much-deserved point.
The draw stretched Millwall's unbeaten run to four games, and left the Lions still without a home win since January.