Goals from James Beattie, Gary Cahill and Chris Armstrong sealed all three points for the Blades as they moved up to 13th place in the Championship.
When these two teams last met in April, Premier League survival was on the line but neither could seize the initiative and a 1-1 draw in the relegation six-pointer was not enough to save either team from the drop the following month.
Both sides have endured contrasting starts to life in the Championship but struggling United showed the most attacking intent early on as the midfield combination of Armstrong and David Carney threatened with dangerous crosses.
The home side soon settled and, despite opting to use Chris Iwelumo as the only striker, began to threaten themselves.
Winger Lloyd Sam's 20-yard shot forced Blades keeper Paddy Kenny into action in the 13th minute and then tried again from the same distance six minutes later with no success.
But it was not until the 34th minute that the deadlock was broken when Jerome Thomas was adjudged to have brought down Carney in the area.
Talismanic striker Beattie stepped up to take the penalty and coolly dispatched to score his 12th goal of the season.
Charlton boss Alan Pardew was clearly unimpressed with his team's first-half performance and rang the changes after the interval.
Veteran midfielder Matt Holland came on for only his third appearance of the season in place of the ineffective Jose Vitor Semedo while weekend goalscorer Luke Varney replaced Sam as the Addicks switched to a 4-4-2 formation.
Former Crewe man Varney made an immediate impact and his 49th-minute header from captain Andy Reid's cross whistled narrowly wide.
Front pairing Iwelumo and Varney began to link up well and summer signing Iwelumo saw his header sail over seven minutes later.
But the home side could find no way past United goalkeeper Kenny who was in inspired form as he pulled of a string of fine saves to deny Danny Mills and then Iwelumo before Zheng Zhi cleared the crossbar with the the goal at his mercy moments later.
And it was rough justice for Charlton as the visitors went down the other end and doubled their lead minutes later.
On-loan Aston Villa defender Cahill was on hand to slot home the loose ball from the left side of the area in the 75th minute to make it 2-0.
Charlton have developed a reputation for scoring late goals this season and manager Pardew urged his men forward from the touchline in order to salvage something from the game.
Armstrong added made it 3-0 in the 88th minute as he latched on to Beattie's pass before producing a consummate finish from the right side of the area.