A vehicular exterior rearview mirror assembly includes a mirror head accommodating a mirror reflective element. An actuator is electrically operable to move the mirror head relative to a mounting base between a folded position and an extended position. The actuator includes a base portion that attaches at the mounting base and a pivot tube that extends from the base portion. The actuator includes a biasing mechanism that, when the mirror moves between the extended and folded positions, biases the mirror head relative to the mounting base in a direction parallel to a longitudinal axis of the pivot tube. The biasing mechanism includes a shell fixed relative to the mounting base, a longitudinally movable portion coupled to the mirror head and a torsion spring that biases the longitudinally movable portion in the direction parallel to the longitudinal axis.