A retrofit luminaire assembly is provided for retrofitting a mounting frame such as a plaster frame or other recessed lighting assembly without the need to remove the existing plaster frame. The retrofit assembly includes a mounting ring that can support a replacement electrical junction box and the replacement luminaire. The mounting ring is inserted through the opening in the plaster frame and secured to the plaster frame by an attachment member that engages the top face of the plaster frame. The attachment member can include a plate coupled to the mounting ring to form two outwardly extending arms. The plate is axially adjustable with respect to the mounting ring to clamp the plaster frame between the plate and the bottom flange of the mounting ring. In another embodiment, the attachment member has a hook-like end that engages a bottom surface of the mounting frame.