A single-frequency two-stage amplifier delivering up to 44 mJ at 2.1 μm is demonstrated. The first-stage amplifier is double-end-pumped by a 1.9 μm Tm:YLF laser, and is seeded with a 15 mJ single-frequency Ho:YAG laser operating at 200 Hz. The second-stage amplifier is end-pumped by another 1.9 μm Tm:YLF laser. The output pulse width from the second-stage amplifier is 113 ns, with a beam quality of M2∼1.32.