The prolific forward struck for the 25th time this season when notching the only goal of the game in the 25th minute.
Mackail-Smith collected a pass from strike-partner Aaron McLean before clipping the ball over the advancing County keeper Conrad Logan from 12 yards.
Promotion-chasing Posh were the dominant team in an opening half in which wideman George Boyd terrorised the visiting defence.
Only four minutes had past when Boyd breezed past two County defenders before steering a shot wide.
That set the tone for wave after wave of home attacks. Boyd drilled an angled effort wide before midfielder Paul Coutts saw an effort blocked by defender Josh Thompson.
Only a fine save from Logan prevented Dean Keates from scoring with a close-range header after more Boyd magic, but even the County shot-stopper was powerless when Mackail-Smith struck.
Boyd gave 16-year-old County full-back Andrew Halls such a torrid time that boss Jim Gannon replaced him after only 33 minutes.
Posh should have doubled their lead five minutes before the break when Boyd was denied by Logan and Dominic Green rolled the rebound against an upright before Thompson hacked clear.
County offered no attacking threat during the opening period but that changed after the interval.
Posh keeper Joe Lewis could only parry a Leon McSweeney free-kick into the path of substitute Olli Johnson, who thundered an effort against the bar from eight yards.
But Posh did regain their control and Mackail-Smith sent an overhead kick straight at Logan after substitute Chris Whelpdale had headed down a Coutts corner.
Whelpdale then thundered a 30-yard effort against a post before Boyd hooked over on the turn, but one goal ultimately proved to be enough.