Charlton looked to have sealed an easy win when Benson's strike a couple of minutes after half-time added to first-half efforts from Johnnie Jackson and Joe Anyinsah.
However the home side came back from the dead after on-loan Leeds forward Mike Grella had thrown them a 58th-minute lifeline.
Further goals from Craig Curran and Gary Madine had Charlton rocking before Benson popped up with his late decider.
Jackson fired Charlton ahead in the 23rd minute when he got on the end of Scott Wagstaff's cross, and former United striker Anyinsah doubled their lead seven minutes before the break with a close-range finish.
Benson put them three up with a 10-yard drive, before Grella got Carlisle started as he forced a strike in after a Ben Marshall header.
Madine then crossed for Curran to head home on 61 minutes and the comeback was complete with 13 minutes left when Madine bundled in after a Tom Taiwo cross.
But just as it looked as though the scoring was over, Benson thumped home a header from Simon Francis's corner on the right.