A holster system that comprises a holster and an accessory configured to be fixed to a mounting rail of a handgun. The holster has a holster body with a pair of opposing wall portions defining a cavity. Each wall portion has an inwardly projecting rib dividing the cavity into an upper first cavity portion and a lower second cavity portion. The first cavity portion is dimensioned to receive various handgun makes and models in a spaced relationship from three sides thereof. The second cavity portion is configured to receive the accessory so that a conforming engagement is formed between the accessory and the holster body. A stop surface of the holster body engages a forward facing surface of the accessory upon insertion of the handgun with accessory into the holster body thereby providing a seating position of the handgun and accessory in the holster body. A retention mechanism of the system has a blocking portion movable between a blocking position and a non-blocking position. The blocking member at least inhibits removal of the handgun and accessory if the handgun and accessory are urged rearwardly before a thumb receiving portion is depressed. The blocking portion is on a spring member that is deflected by an elongate exteriorly extending sliding member that has a cam surface that deflects the spring member from the blocking position.