A rear-loaded injector cartridge for an intraocular lens (IOL) having a proximal opening that provides a haptic slot. The haptic slot receives a leading haptic of an IOL loaded therein, and temporarily retains the leading haptic while the optic of the IOL is inserted into a holding area of the cartridge. As the optic passes by, the leading haptic folds over the top of the optic, on its anterior side. The length of the haptic retention slot is sufficient to maintain the leading haptic in its anteriorly folded position while the IOL remains in holding area, typically while the cartridge is mated with a handpiece of the injector. The cartridge also has a rear or proximal cut out which advantageously keys with a similarly-sized rail on the handpiece so that the cartridge cannot be inserted in the wrong way.