A needle mounting assembly for mounting a pen needle in fluid communication with a medication cartridge. A collet defines an opening in which is slidably fittable the outlet end of the cartridge. The collet is shiftable relative to the cartridge outlet end. When the collet is disposed in a first axial position, a plurality of fingers of the collet extend forward of the cartridge outlet end and are disposed in a first radial arrangement in which a needle hub of the pen needle can be freely axially placed onto and freely axially removed from the plurality of fingers. When the collet is moved to a second position, the plurality of fingers are shifted by engagement with the cartridge outlet end to operationally secure the needle hub. When the collet is moved back to the first position, the plurality of fingers shift back for free axial removal of the needle hub from the mounting assembly.