But it was hard work in a game dominated by the home team in the first half.
There was a great chance for Hartlepool to take an early lead when Millwall defender Tony Craig was judged to have handled Joel Porter's flick on the fringe of the penalty area.
Porter struck his penalty well, but keeper David Forde guessed right and dived to his right to push the ball away.
But Hartlepool still had a shock in store for Millwall, taking control of the game with two stunning goals in two minutes, both scored by midfielder Antony Sweeney.
First, on 23 minutes, he volleyed in a spectacular looping shot from 25 yards as a Millwall clearance landed at his feet.
Then he provided the finish from ten yards after he and Porter had raced from their own half on a great break.
Millwall staged a brilliant second half fight back, led by sub Harris, who grabbed a quick fire hat-trick to give his team the lead.
He narrowed the gap on 62 minutes when he curled a 20-yard free-kick into the top corner of the net, and the home defence started to panic.
It got worse for them five minutes later when Harris cut inside from wide on the left and scored with a low shot from the tightest of angles.
His hat-trick came on 72 minutes when Sam Collins made a mess of a clearance just inside the home area, and Harris gleefully smashed in his shot.