A novel dopamine (DA) sensor based on a polymerized mediator (PEI-Fc) and sulfo-group-functionalized graphene oxide (PEI-Fc/SFGO) composite modified electrode was developed. The PEI-Fc/SFGO composite was formed by electrostatic interaction, in which ferrocene (Fc) groups served as an electron transfer mediator to improve the sensitivity. Scanning electron microscopy (SEM) was employed to characterize the surface morphology of the PEI-Fc/SFGO composite. Ultraviolet visible spectroscopy (UV–vis) and Fourier transform infrared (FTIR) spectroscopy were performed to characterize the PEI-Fc and SFGO, respectively. Cyclic voltammetry (CV) was employed to investigate the electrochemical behavior of the PEI-Fc/SFGO composite modified electrode. Linear sweep voltammetry (LSV) was employed for the quantitative detection of DA. The as-prepared sensor realized a wide linear range of 0.5–100 μM with a low limit of detection (LOD) of 0.1 μM for the determination of DA. This work provides a simple surface modification technique and a potential electrode material for the detection of DA.