After a quiet first half tempers flared in the second when Millwall's Jimmy Abdou was sent off for a horror tackle on Clint Easton and Hereford's Nick Chadwick followed suit.
But it was Grabban who stole the headlines when he rounded off a bursting drive at the second attempt to put Millwall's promotion charge back on track.
On-loan Leeds striker Tresor Kandol twice failed to hit the target early on, with Chris Hackett also offering promise on the right when he gifted Neil Harris a golden opportunity after 33 minutes, but the 31-year-old watched the ball sail past his nose.
Things turned sour immediately after the restart following Abdou's double-footed lunge and Chadwick's second yellow on Kandol.
Bradley Hudson-Odoi came close for the Bulls, but they had no answer to Grabban who after an earlier miss lashed home late on after stinging the gloves of goalkeeper Darren Randolph.