An adjustable dressing mirror assembly includes a support, a mirror-mounting unit, and a positioning device. The positioning device includes a guiding seat connected pivotally to the support, a driving plate connected pivotally to the mirror-mounting unit, a sliding block, a movable member, a positioning rod, and a resilient member. The guiding seat includes an accommodating chamber and a plurality of positioning grooves. The driving plate and the sliding block are movable in the accommodating chamber. The positioning rod is disposed fixedly on the movable member. Engagement between the positioning rod and the positioning grooves is allowed with assistance of the resilient member during movement of the driving plate within the accommodating chamber from a rear limit position to a front limit position, and is prevented during movement of the driving plate within the accommodating chamber from the front limit position to the rear limit position.