The Lions finally ended a run of five games without a goal thanks to Liam Feeney's fortuitous second-half goal.
But they failed to register their first away win of the season or even take a point as the hosts deservedly fought back through Carl Dickinson and Craig Forsyth.
Kenny Jackett's men ended their barren run in front of goal when Feeney's speculative cross-cum-shot looped off Lloyd Doyley and into the net.
The home side deservedly drew level after 78 minutes as Dickinson drove into the bottom corner from 20 yards.
There was more to come for the resurgent Hornets as Millwall failed to clear from a corner and Forsyth was perfectly placed to drive the ball in with just five minutes to play.