The game got off to a flying start with two goals inside the opening six minutes.
Woodrow put the hosts ahead in only the second minute, stabbing the ball in from close range after Alex Mowatt's free-kick into the area caused problems for the Millwall defence.
The game then settled down for a period with the two teams cancelling each other out and neither side managing to create a chance of note.
Towards the latter stages of the half, Barnsley's Carlton Morris forced his way into the area and Shaun Hutchinson did well to make an important block which saw the striker's deflected effort tipped over by Bartosz Bialkowski.
Collins made a comfortable save after Bennett met a Shaun Williams cross with a header.
Bialkowski was then called into action at the other end, tipping over Mowatt's free-kick and also keeping out an effort from Woodrow.
Helik put Barnsley back in front in the 59th minute, blasting a close-range volley into the roof of the net.
Millwall had a chance to draw level in the 73rd minute when Bennett found himself with just the keeper to beat but he fired over.
Barnsley appeared content to see out the game and protect their lead in the closing stages, with Millwall never seriously threatening to snatch a late leveller which would have extended their eight-match unbeaten run.