<p>Superconducting qubits possess outstanding capabilities for processing quantum information in the microwave domain; however they have limited coherence times. An interface between photons and phonons could allow quantum information to be stored in long-lived mechanical oscillators. Here, we introduce a platform that relies on electrostatic forces in nanoscale structures to achieve strong coupling between a superconducting qubit and a nanomechanical oscillator with an energy decay time (<i>T</i>₁) of approximately 25 ms, well beyond those achieved in integrated superconducting circuits. We use quantum operations in this system to investigate the microscopic origins of mechanical decoherence and mitigate its impact. By using two-pulse dynamical decoupling sequences, we can extend the coherence time (<i>T</i>₂) from 64 μs to 1 ms. These findings establish that mechanical oscillators can act as quantum memories for superconducting devices, with potential future applications in quantum computing, sensing and transduction.</p>