Described herein is a pre-mounted lock assembly for a door of a low and medium voltage electrical cabinet including a containment profile that develops along a main direction defining a seat. A mechanism is arranged in said seat. Said mechanism includes at least a rod and at least one guide element for said rod. Said guide element is rigidly connected to said profile. The pre-mounted lock assembly also includes fixing means for fixing it to said door following the connection of said at least one guide element to said profile, wherein said fixing means are arranged on said profile and/or on said at least one guide element.