Lys Mousset completed 90 minutes, but the Blades were on the wrong end of an entertaining seven-goal contest at Bramall Lane.
Kieron Freeman also played a full game, but despite goals from Reece York, Callum Gribbin and Sammy Ompreon and an encouraging attack display, it was Charlton who collected the three points.
Boyes had United's first effort at goal, firing low and hard from the edge of the box but Harness was just about equal to his effort. Moments later, Boyes was again in the thick of the action, crossing to York, who headed over unmarked from a promising position.
On 15 minutes the Blades should have hit the front. The ball fell kindly for Mousset, with Ompreon involved in the build-up, but Harness was quickly off his line to deny the advancing Frenchman. Mousset and the Addicks stopper continued their battle soon after, this time United's number nine saw an acrobatic volley turned wide of the post for a corner.
United's persistent pressure reap its reward on 21 minutes, as YORK applied the finishing touch after Mousset's second attempt from a corner looped over Harness with the aid of a deflection. But Charlton hit straight, restoring parity within 60 seconds, as MORGAN fired past Amissah from distance, although the United stopper was wrongfooted by an unfortunate deflection off York.
Mousset continued to look a live threat and his chip pass played in Freeman, but he lifted an effort over under pressure from Harness. Then against the run of play, the visitors went in front, with MORGAN firing home his second of the afternoon from the penalty spot, after Hall had been adjudged to have fouled Isiaka.
The Blades were quickly out of the blocks in the second period, although Mousset could have perhaps shot instead of square the ball to Ompreon, who saw a shot blocked, before Mousset was denied at the back post in the second phase. But from the resulting corner, United levelled, as trialist GRIBBIN showed excellent composure, before benefiting from another deflection which left Harness helpless.
But just like in the first-half, Charlton netted within a minute of the restart. Clayden got the better of his marker down the left, before FERGUSON was unfortunate to divert the ball into his own net past Amissah at the near post. The game continued to be an open contest and Ompreon drew a save from Harness having beat the offside trap.
Both teams were creating chances on a regular basis, this time Amissah kept the Blades in the contest with a double save to deny Powell. At the other end, York fashioned an opening and United levelled for a second time, as OMPREON fired past Harness at the Kop end, before being replaced by Young.
United made a second change, as York was withdrawn and replaced by trialist Hoti, but fell behind again on 66 minutes as POWELL forced his way through the home defence before squeezing the ball past Amissah. Looking for another reply, the Blades attacked with purpose down the left, but Ferguson fired over left-footed from inside the box.
With the clock ticking down United pushed for a deserved point, but the Addicks held firm to claim the spoils.
Blades: Amissah, K. Freeman, Boyes, Ferguson, Gordon, Hall, Broadbent, Gribbin (Brunt 76), Mousset, Ompreon (Young 62), York (Hoti 64)
Unused: Kelly, Mallon.