Aberdeen had goalkeeper Jamie Langfield dismissed in the incident that led to the opening goal from Commons before Forrest used his pace to settle the issue three minutes from the end.
Celtic carved out the first opportunity when Joe Ledley played a one-two with Scott Brown and twisted into the penalty area before his curling shot was pushed away by the diving Langfield on 29 minutes.
Commons then cushioned a volley to the back post from a deep Adam Matthews delivery, but Brown was unable to squeeze the ball on target from a tight angle.
But the hosts were left facing an uphill task on the stroke of half-time when Georgios Samaras held off the challenge of Russell Anderson and rounded Langfield, who whipped the feet from the Celtic striker.
Langfield was dismissed and a couple of minutes later Commons blasted the resulting spot-kick under replacement keeper Nicky Weaver.
Aberdeen should have levelled the match when Niall McGinn rose between Steven Mouyokolo and Emilio Izaguirre to meet a Jonny Hayes cross but his header was brilliantly blocked by Fraser Forster.
Either side of that chance Samaras volleyed over from six yards after some head tennis from a corner and Charlie Mulgrew curled a drive from the edge of the box narrowly wide of the target.
Brown dragged a shot wide of the post after a Commons cut-back was dummied by Samaras and the big Greek later set up Ledley, whose low drive was blocked by the legs of Weaver.
Celtic were comfortably seeing the game out, but they made sure of the points when substitute Forrest (87) used his pace to cut into the box from the left flank and his shot took a nick off Mark Reynolds and looped over Weaver into the net.