Mackay struck the winner 16 minutes from time as Oldham's hopes of staging a run into the League One play-offs took a major blow.
The Latics were unbeaten in five previous home games but their lack of ideas going forward held them back as Hartlepool defended with organisation and strength.
Both teams suffered injury set-backs with Hartlepool losing top-scorer Richie Barker in the first half, quickly followed by Oldham's on-loan striker Jordan Robertson on his full home debut for the club.
In a game of very few clear chances, the first arrived after less than two minutes when Andy Monkhouse was given acres of space in the Oldham box.
Monkhouse steadied himself ten yards out but was denied by veteran goalkeeper Mark Crossley, who was quickly off his line to smother.
Oldham midfielder Gary McDonald hit the side-netting from a difficult angle before striker Lewis Alessandra had a 20-yard shot deflected off target.
By that point Barker had already been forced off and Robertson did not last much longer - he was replaced by another loan striker in Leon Constantine.
Oldham's Chris Taylor blasted just past the far post early in the second half and Alessandra gave visiting keeper Arran Lee-Barrett his first test with a shot from 20 yards.
But Hartlepool showed their threat as Richie Humphreys crossed and Monkhouse planted his header goalwards only for Crossley to make a fine save.
The visitors broke through when Mackay beat the offside trap to race clear to the left of goal and send a superb chip over Crossley's head from just outside the penalty area.
Oldham's only chance to equalise fell to young substitute Deane Smalley but he drove straight at Lee-Barrett as Oldham faded towards defeat.