A holster for receiving and withdrawing a handgun has a proximal wall portion with a handgun retention mechanism thereon. The mechanism having a first lever with a tab portion for actuation and release of a trigger guard, the lever having a snap fit into a lever region without a separate pin. A belt engaging member covers the lever region capturing the first lever therein. If a second retention mechanism is used, another lever with a tab portion is positioned so the tabs are stacked, depressing one also depresses the tab behind it, such that both mechanisms are released. Overmolded polymer layers in the holster body provides a softer interior handgun engagement surfaces. The holster body is split forwardly with a threaded fastener for clamping onto the handgun. No holster body is beneath or rearward of the tab portion, configured as a thumb button, allowing smooth transition to gripping the handgun.