A universal magazine release mechanism for firearms has a frame defining a cartridge magazine chamber receiving a cartridge magazine and having a magazine release receptacle intersecting the cartridge magazine chamber. A magazine release member is mounted within the magazine release receptacle and has linear movement to a magazine retaining position and a magazine release position. A spring urges the magazine release member toward the magazine retaining position. A magazine release button extends from the magazine release member and is exposed externally of the firearm frame for engagement and linear movement by the thumb of a user's hand. The magazine release member is selectively positioned to locate the magazine release button for thumb actuation by a right-handed user or a left-handed user.