A sensitive electrochemical platform was fabricated based on a nanocomposite for the simultaneous electrochemical detection of dopamine (DA) and ascorbic acid (AA). The electrodeposition of Au nanoparticles (AuNPs) was performed on an electrochemically reduced graphene oxide (EDGO)-modified glass carbon electrode (GCE). The modified electrode was further functionalized with mercapto-β-cyclodextrin (HS-β-CD), which produced the β-CD/AuNPs/EDGO modified electrode. The electrochemical behaviors of DA and AA were investigated by cyclic voltammetry on the surface of β-CD/AuNPs/EDGO-modified electrodes. The separation of oxidation peak potentials for AA and DA was about 182 mV, which allowed simultaneous determination of AA and DA. Differential pulse voltammetry was used for determining AA and DA in their mixture at the β-CD/AuNPs/EDGO-modified electrode. Under optimum conditions, the linear response ranges for determination of AA and DA were 50–900 μM and 0.5–120.0 μM, with detection limits (S/N = 3) of 2 μM and 0.024 μM, respectively. Good stability and high resistance to interference were further demonstrated for the as-prepared electrochemical sensor.