A second-half scorcher from Antony Sweeney saw goal-shy Millwall slide to a second successive defeat.
Sweeney's strike five minutes after the break proved decisive in a scrappy game that saw The Lions fail to score for the first time in four matches.
It was an afternoon of total frustration for the home side, compounded by Millwall playing the last five minutes with 10 men after Danny Senda limped off injured.
The Lions were dealt a blow before kick-off when in-form striker Will Hoskins was unable to take his place in the starting line-up with a hamstring injury picked up in the match at Leeds last Saturday.
Jay Simpson stepped in to fill the vacant striking berth alongside gary Alexander, whilst loan signings Ahmet Brkovic and Bryan Hodge were both handed their home debuts.
It was the visitors who started brightly though, with Gary Liddle and then Joel Porter both dragging efforts wide from good positions.
A bit of Brkovic magic provided Millwall's first shot on goal after four minutes, and the midfielder again went close following another superb piece of skill after 24 minutes. On this occasion, his dipping low drive was gathered at the near post by keeper Jan Budtz.
Alexander headed narrowly wide from a Senda delivery on 29 minutes, with Lenny Pidgeley scrambling across his goal to hold a shot from Porter as Hartlepool threatened.
Millwall should have gone in at half-time a goal, first when Alan Dunne headed wide after O'Hara's freekick picked him out unmarked eight yards from goal, and then when Alexander's header was pushed onto the post by keeper Budtz.
Brkovic had two half-decent chances to open his account early in the second-half with an effort tipped over the bar and then a right-foot shot from Alexander's flick-on that flew inches wide.
But two minutes later it was the visitors who had made the all-important breakthrough, courtesy of a sublime 20-yard finish from Sweeney into the roof of the net.
Hartlepool could have added two further goals in quick succession when Liddle's free-kick dropped inches over and then James Brown fired wide from close range.
Caretaker assistant boss Colin West brought on Ben May for debutant Hodge as The Lions switched to a three-pronged attack.
But it was midfielder Brkovic who posed the biggest threat and he was denied once again on 67 minutes, this time his effort was blocked on the line after Simpson's trickery had created the opening.
The introduction of Zak Whitbread for Shaw and Neil Harris for Simpson in the final 10 minutes was the last throw of the dice, but when Senda pulled up injured as he stretched for a challenge, it left the Lions both a man - and a goal - down. It was one of those afternoons when quite simply nothing went Millwall's way.

Millwall: Pidgeley, Senda, Frampton, Robinson, Shaw (Whitbread, 82mins), Dunne, O'Hara, Alexander, Simpson (Harris, 82mins), Brkovic, Hodge (May, 63mins). Subs not used: Day, Fuseini.
Booking: Alexander (24mins)

Hartlepool United: Budtz, Gibb, Nelson, Clark, Liddle, Humphreys, Barker, Porter (69mins), Monkhouse, Brown, Sweeney. Subs not used: Lee-Barrett, Elliott, Antwi, Moore, Mackay.
Bookings: Gibb (29mins), Monkhouse (35mins)
Scorer: Sweeney (50mins)

Ref: Steven Cook