And to add to the Bramall Lane side's misery midfielder Stephen Quinn was red-carded in the dying minutes for a crude tackle on sub Jose Semedo, as goals from Chris Iwelumo and Sam Sodje sank Kevin Blackwell's side.
Charlton, still smarting from an embarrassing 5-3 defeat at Blackpool last week, almost snatched an early opener, United failing to cut out full-back Greg Halford's long throw as Jerome Thomas stuck out a foot to stab the ball inches wide from six yards.
United were still dozing when China international Zheng Zhi reacted quickly to Darren Ambrose's teasing free-kick to hook the ball over the target, as Charlton's five-man midfield quickly took control.
Paddy Kenny, exonerated over the own goal fiasco at Middlesbrough, pulled off a stunning save, snatching Luke Varney's crisp strike at the foot of the post after neat work from Iwelumo.
The Sheffield side, struggling to find their rhythm, were unlucky when Billy Sharp just failed to connect with a Quinn cross, before Kenny pulled off another spectacular stop this time keeping out Matt Holland's low drive from the edge of the box.
Veteran midfielder Gary Speed saw his long-range strike flash over the bar, before Iwelumo struck deep into first-half injury time. Another Halford long throw caused confusion in the area as the lanky striker toe-poked the ball past Kenny from close range.
Sharp had a chance to put United back on level terms, but steered his tame header straight to the waiting Nicky Weaver.
Sodje then killed off United on 82 minutes, stretching to reach Darren Ambrose's corner to head past Kenny from close range to ensure that the Addicks travelled home with all three points.