Rumoured Derby transfer-target Mackie fired home a low drive from the edge of the penalty area after Luke Summerfield's shot had been beaten away by the opposition's defensive wall.
Argyle's previous home match had been postponed because of waterlogging, and the bobbly pitch still showed the scars of its earlier pummelling by the elements.
Slick passing football was at a premium and, consequently, chances at either end were few and far between.
Recalled Derby striker Rob Hulse saw a shot deflected over by Karl Duguid from six yards, while Argyle captain Carl Fletcher drove a shot over the Derby crossbar after the visitors had failed to deal properly with a long throw-in from Kari Arnason.
The home side had the game's first shot on target, after half-an-hour, when midfielder Luke Summerfield thumped in a drive from 25 yards that County goalkeeper Stephen Bywater clutched easily to his chest.
Arnason came close to breaking the stalemate just before the interval, although at the wrong end, from his point of view.
The Iceland international defender intervened to clear Paul Green's low cross from the right although he did not appear to have much control over the ball, which shaved the Argyle goalpost as it went off for a corner.
The second half opened with Hulse clipping Stephen Pearson's sweet pass just over David Stockdale's crossbar, but most of the attacking intent that followed came from the home side.
Mackie fired over after Summerfield's defence-splitting pass, while Rory Fallon nearly capitalised on Alan Judge's low curling cross.
Arnason went close with a header from Summerfield's corner, while Hulse nearly steered home a shot at the near post, before Green slammed a shot against the Argyle post from only a few yards out after substitute Steve Davies' shot had been blocked by Stockdale.
Hulse was thwarted by Stockdale's trailing leg after he wriggled to make room in the Pilgrims' penalty area before Mackie struck to decide the match.