A vehicular interior rearview mirror assembly includes a mirror head that accommodates an electronic component and a heatsink assembly. The heatsink assembly includes (i) a first portion that is thermally coupled to the electronic component and draws heat from the electronic component, (ii) a second portion that extends from the first portion, (iii) a chamber extending between the first and second portions and (iv) a phase-changing refrigerant within the chamber. With the refrigerant in a liquid phase and responsive to heating of the first portion, the refrigerant draws heat from the first portion and changes to a gaseous phase. With the refrigerant in the gaseous phase, the refrigerant flows within the chamber and along the second portion and condenses at an inner surface of the second portion to transfer heat to the second portion. The heat is dissipated from the second portion to exterior the mirror head.