A day/night rearview mirror assembly for vehicles including a one-piece, molded, resinous mirror case having flexible, resilient portions and a substantially rigid pivot actuator which is biased by the case into either a day or night position. Movement of the actuator changes the position of the case and mirror element thereby changing the amount of light reflected. An interference fit between supports for the actuator within the case and the pivot actuator expands and pre-stresses the case to provide a predetermined biasing force which must be overcome by a pivoting force to move the actuator between its positions. A continuous lip extending around the periphery of the case allows snap-in insertion and permanent retention of a preferably prismatic mirror element. The retaining lip is sized to maintain coverage of the edge of the mirror element even during flexing of the case while the actuator is pivoted.