Time-bin photonic qubits are well-suited for quantum network applications due to their robustness to polarization instability in fiber links and potential for heterogeneous networks. In this work, we implement the first entanglement-preserving polarization-to-time-bin conversion of a photon qubit in an entangled state with a matter qubit. Photons initially generated with polarization encoding are converted to the time-bin basis through a polarization-discriminating asymmetric Mach-Zehnder interferometer. The photonic qubits are generated via the 1092 nm transition of a ^88Sr^+ ion. We measure state fidelity bounds of 0.906 ± 0.011 ≤ℱ≤ 0.934± 0.011, with conversion error < 0.028, and find this fidelity is unaffected by depolarizing noise even at full depolarization strength.