There was only one change in the Bluebirds line-up from the side that drew at Blackpool with Paul Parry out wide in place of the injured Joe Ledley.
Charlton had two early strikes with Matt Holland firing across goal and Hameur Bouazza rippling the side-netting as the visitors impressed on the break.
Bouazza beat Kevin McNaughton down the left and Roger Johnson did well to head over his own bar with Andy Gray waiting at the far post.
Bouazza was opening up the home defence and in the 12th minute he slotted a superb pass into Luke Varney, but the former Crewe striker stumbled in front of an open goal and the chance was lost.
City took the lead against the run of play in the 18th minute when Peter Whittingham made space for a shot.
His low drive was pushed out by Nicky Weaver to McCormack, who hit a first-time shot into the back of the net.
The goal settled City but the visitors were still dangerous coming out of defence with Darren Ambrose and Bouazza both busy as Charlton controlled midfield.
The Addicks made one change for the second half with Jose Semedo replacing Martin Cranie at full-back.
But the substitute lasted only seven minutes before being red carded after bringing McCormack down well inside the penalty area.
To everyone's amazement, referee Kevin Friend gave the Bluebirds a free-kick just outside the box.
It made little difference, however, as McCormack struck the free-kick into the corner of the net for his ninth goal of the season.
A Bouzza cross in the 63rd minute gave Linvoy Primus a clear heading chance but he lofted the ball high and wide.
Charlton were down to nine men when skipper Mark Hudson received two yellow cards in the space of two minutes.
He brought down Parry after 66 minutes and then downed Jay Bothroyd in the 68th minute.
It was one-way traffic from then one but the Bluebirds were unable to make the extra men count despite being in complete control.