There is a growing interest in utilising diamond-related electrodes for biomedical applications, like cellular stimulation and tissue engineering, due to their exceptional biostability and long–term durability when implanted into living tissue. Here, we investigate the electrochemical performance and stability of laser-graphitised polycrystalline diamond (PCD) as an electrode and evaluate its biocompatibility with human mesenchymal stem cells (hMSCs). The PCD surface was graphitised using an ultraviolet (UV) nanosecond-pulsed laser under ambient atmosphere and temperature conditions. Microstructural changes were analysed before and after laser writing using scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrochemical performance and stability were evaluated through Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), and multi-potential and multi-current steps over an extended period of time. To study the electrode’s biocompatibility, bone marrow hMSCs were cultured on the surface of the electrode and tissue culture plastic (TCP) as controls, followed by a resazurin assay at 24, 48, and 72 hours. The laser-graphitised electrode exhibited a high specific double-layer capacitance of 195 µF/cm² and a specific electrochemical impedance of 129 Ω·cm² at 1 kHz. The hMSCs cultured on the electrode showed over 80% viability relative to TCP, indicating the non-cytotoxicity and growth-promoting effect of hMSCs. Notably, the hMSCs cultured on the electrode were elongated after 3 days, aligning with the topography of the laser-graphitised PCD surface. The graphitisation of PCD through nanosecond laser writing presents promising electrochemical properties, indicating its suitability as both a recording electrode for biosensing and a stimulating electrode for cellular modulation. Furthermore, the laser-graphitised PCD demonstrated notable biocompatibility with hMSCs. These findings suggest its potential for various biomedical applications, including biosensing and stem cell therapeutics, through promoting the proliferation and guiding the differentiation of stem cells into specialised cell types.