A vehicular exterior rearview mirror assembly includes a mirror head having a mirror casing and a mirror reflective element, a support structure configured for attachment at a side of a vehicle, with the mirror head adjustably mounted at the support structure. The support structure extends laterally from the side of the vehicle. An adjustment mechanism is disposed at the end of the support structure. The adjustment mechanism is movably received at the mirror head and includes a rotatably driven gear that engages a toothed track of a mirror head bracket. Rotation of the gear causes translational movement of the mirror head bracket and the mirror head relative to the support structure to adjust the mirror head position relative to the support structure between a retracted position and an extended position.