A firearm mounting plate assembly is provided. The firearm mounting plate assembly includes a firearm slide with a first side, a second side, and a top side. A recess defined by a first slide wall, a second slide wall, and a recess base surface is disposed on the firearm slide top side. The recess includes a rib, a slot, and an ejector projection. The firearm mounting plate assembly includes a mounting plate with a top surface, a bottom surface, a front side, and a rear side. An ejector notch and a channel is disposed along the bottom surface. The mounting plate includes a stud, and one or more bosses. The mounting plate is configured to secure within the recess of the firearm slide.