Leon Cort's 82nd-minute header amid a goalmouth scramble looked to have handed the Addicks all three points at St Andrew's in what was their first game back in the Championship after a three-year hiatus.
But last season's play-off semi-finalists rescued a point with almost the last kick of the game, coming after Zigic - linked with a summer move away from the club - had only been brought on in the aftermath of Cort's header.
The encounter appeared to have been heading for a goalless draw, with England goalkeeper Jack Butland, making his long-awaited competitive bow for Blues, and Charlton counterpart Ben Hamer never seriously threatened during an encounter dominated by a lack of quality in the final third.
But, on reflection, both sides will be happy with a point, with Charlton proving they can certainly mix it with the division's leading promotion contenders.
Butland's inclusion came after a summer in which the 19-year-old was part of England's Euro 2012 squad, selected as number one for Team GB at the Olympics and then made his debut for the Three Lions against Italy in midweek.
Butland's absence for City's 5-1 Capital One Cup mauling of Barnet on Tuesday meant boss Lee Clark made just the one change in what was his first league game since taking over from Chris Hughton in the summer.
Opposite number Chris Powell recalled his strongest XI for the league opener, making eight changes from the side knocked out of the League Cup on penalties by Leyton Orient.
After a lively and balanced opening 15 minutes, it was Charlton who created the first real opening as the recalled Yann Kermorgant somehow headed over Johnnie Jackson's cross from just a couple of yards out.
Peter Lovenkrands fired wide for Blues at the other end before Darren Ambrose, booed with every touch against one of his former employers, drilled a free-kick straight at Hamer.
Both sides were looking threatening albeit without really testing either 'keeper, Jackson heading over for the Addicks from Rhoys Wiggins' cross.
Ravel Morrison fired narrowly off target for the home side while Kermorgant forced Butland into his first save, a routine one at that, in the 43rd minute.
It was only the sixth attempt on target in an opening 45 minutes which provided 13 shots on goal.
And Chris Burke continued where the game left off as his shot flew just over following the restart.
Jackson met Bradley Pritchard's cross but again the Charlton skipper headed over from a promising position.
Marlon King's effort failed to trouble Hamer while hot prospect Nathan Redmond, brought on to add fresh impetus to Birmingham's attack, blazed wide from six yards in 77th minute.
But Cort finally found the net five minutes later after the hosts had failed to clear Pritchard's cross.
The strike looked to have won it for Charlton until Zigic, fresh off the bench, kept his cool to fire home from 12 yards deep into injury time.