It took 24 minutes for either keeper to be tested when MK Dons stopper David Martin was forced to save Jose Semedo's powerful shot.
Benson capitalised on a mistake by Danny Woodards eight minutes later but the Addicks striker was stopped in his tracks by the defender on the edge of the area, an offence which earned the Dons player a yellow card.
Benson came close to heading Charlton in front on the stroke of half-time but his effort from Simon Francis' cross went just wide.
Aaron Wilbraham headed just over from close range after rising to meet Peter Leven's 52nd-minute free-kick in a rare Dons attack, but a shocking lack of quality from both sides ensured real chances remained few and far between until Jonathan Fortune headed just over in the 78th minute.
The deadlock was finally broken five minutes from time when substitute Kyel Reid's cross was drilled home by Benson for his first goal as a Charlton player since his arrival from Dagenham last month.