Engineered dissipation can be used for quantum state preparation. This is achieved with a suitably engineered coupling to a dissipative cold reservoir usually formed by an electromagnetic mode. In the field of cavity electro- and optomechanics, the electromagnetic cavity naturally serves as a cold reservoir for the mechanical mode. Here, we realize the opposite scenario and engineer a mechanical oscillator cooled close to its ground state into a cold dissipative reservoir for microwave photons in a superconducting circuit. By tuning the coupling to this dissipative mechanical reservoir, we demonstrate dynamical backaction control of the microwave field, leading to stimulated emission and maser action. Moreover, the reservoir can function as a useful quantum resource, allowing the implementation of a near-quantum-limited phase-preserving microwave amplifier. Such engineered mechanical dissipation extends the toolbox of quantum manipulation techniques of the microwave field and constitutes a new ingredient for optomechanical protocols.