Sam Sodje powered home an 82-minute header from a corner by Darren Ambrose after Chris Iwelumo had given the visitors the lead with a close-range shot on the stroke of half-time.
United threatened only sporadically, Gary Speed going close towards the end of the first-half with a 25-yard drive and Billy Sharp firing just wide.
The Blades made two changes to the side beaten by Middlesbrough in the FA Cup.
David Cotterill replaced Jon Stead, who was named on the bench, while Chris Armstrong came into the team for Gary Naysmith. Lee Martin is out for up to three weeks with a knee problem.
United's misery was compounded when Stephen Quinn was sent off in second-half injury-time for a reckless lunge on substitute Jose Semedo.
The home team came close on several occasions after the break but were unable to penetrate a well-drilled Charlton defence.
United threatened only sporadically, Gary Speed going close towards the end of the first-half with a 25-yard drive and Billy Sharp firing just wide.
The Blades made two changes to the side beaten by Middlesbrough in the FA Cup.
David Cotterill replaced Jon Stead, who was named on the bench, while Chris Armstrong came into the team for Gary Naysmith. Lee Martin is out for up to three weeks with a knee problem.
United's misery was compounded when Stephen Quinn was sent off in second-half injury-time for a reckless lunge on substitute Jose Semedo.
The home team came close on several occasions after the break but were unable to penetrate a well-drilled Charlton defence.