With leaders Watford also losing, Charlton missed a golden opportunity to make up ground in their promotion chase and this result will have knocked their confidence.
Charlton conceded their first goal in six hours and 51 minutes of football in the 34th minute of the game when Jerome Thomas needlessly bundled David Carney over in the box and James Beattie netted from the spot for his 12th goal of the season.
It was the Blades' first shot on the Charlton goal and they had created very little up to this point.
Charlton had seen an early header from Sam Sodje go over the bar and Lloyd Sam twice put in shots which Paddy Kenny, who was restored to the United goal, save comfortably.
The visitors upped their game after getting in front and Carney saw his shot deflected from a corner.
After the break, Charlton boss Alan Pardew abandoned his 4-5-1 formation and brought on Matt Holland and Luke Varney to give the team a more attacking edge.
But the Yorkshireman defended stubbornly with Kenny again doing well making a double save from Danny Mills and Varney.
Charlton missed the best chance of the match when Varney broke clear on the left and his cross fell to the feet of Chinese international Zhi Zheng, but with only the goalkeeper to beat, he fired over the bar.
It was to prove a costly miss as, with Charlton pushing forward looking for that all important equaliser, United twice broke clear to score two more goals.
On 75 minutes Gary Cahill bundled the ball over the line in a goalmouth scramble and with a minute left Beattie broke clear to put Chris Armstrong in to finish the game in style.