A draw for the Cumbrians finally ended their six-game losing streak but, after leading 3-1, John Ward's side were left disappointed not to be walking off with a victory.
With 94 minutes on the clock the energetic Mackail-Smith ghosted between two defenders to latch onto a lofted through-ball and scuff his shot under Carlisle keeper Ben Alnwick.
The goal had been coming for Darren Ferguson's troops after Mackail-Smith had reduced the deficit with a penalty on the hour.
Graham had opened the scoring for the home side with a composed finish from eight yards after Danny Carlton's clever lay-off.
Posh keeper Joe Lewis had earlier denied Graham with a stunning double save to somehow deny the Cumbrians' top-scorer.
Seconds later, visiting captain Russell Martin cleared off the line from a towering Danny Livesey header as Carlisle threatened an avalanche of early goals.
But Peterborough grabbed their first equaliser with just their second attack on 35 minutes when Charlie Lee was left unmarked from a corner to head past the on-loan Alnwick from close range.
Mackail-Smith should have put the visitors ahead immediately after the break but screwed his shot wide and it was Carlisle who struck next when Simon Hackney's low shot was fumbled by Lewis, leaving Graham a simple finish for his tenth goal of the season.
Within a minute, Carlisle found the net again when Cleveland Taylor's drive was blocked by Lewis only for the rebound to fall to Graham Kavanagh, who skipped past a challenge and finished calmly from 15 yards.
Ten minutes later came Mackail-Smith's penalty after Evan Horwood handled unnecessarily in the area and the visitors could have drawn level but were denied twice by the impressive Alnwick, who saved well from George Boyd and Mackail-Smith yet again.