Recently, spin-transfer torque magnetic cell (STT-mCell) has emerged as a promising spintronic device to be used in Computing-In-Memory (CIM) system. However, it is challenging to guarantee the hardware security of STT-mCell based all spin circuits. In this work, we propose a novel Physical Unclonable Function (PUF) design for STT-mCell based all-spin circuit (All-Spin PUF) exploiting the unique manufacturing process variation (PV) on STT-mCell write latency. A methodology is used to select appropriate logic gates in the all-spin chip to generate a unique identification key. A linear feedback shift register (LFSR) initiates All-Spin PUF and simultaneously generates a 64-bit signature at each clock cycle. Signature generation is stabilized using an automatic write-back technique. In addition, a masking scheme is applied for signature improvement. The uniqueness of the improved signature is 49.61 \(\% \) . With \(\pm 20\% \) supply voltage, and 5°C-105°C temperature variations, All-Spin PUF shows a strong resiliency. In comparison with the state-of-the-art PUFs, our approach can reduce hardware overhead effectively. Finally, the robustness of All-Spin PUF against emerging modeling attack is verified as well.