Graphene nanosheets (GS) were easily prepared from graphite via a one-step ultrasonic exfoliation approach using N-methyl-2-pyrrolidone (NMP) as the solvent. Compared with the widely used graphene oxide (GO) obtained by multistep chemical oxidation, the NMP-exfoliated GS exhibited apparently better electrochemical activity toward the oxidation of a series of phenols like hydroquinone, catechol, 4-chlorophenol, and 4-nitrophenol. Interestingly, the electrochemical activity of GS toward these phenols can be further enhanced by simply anodizing at 1.8 V for 2 min (denoted as EGS), reflected by the apparently enlarged oxidation peak currents in voltammograms and the obviously reduced charge transfer resistance in electrochemical impedance spectra (EIS). Characterizations by techniques like X-ray photoelectron spectra (XPS), Raman spectra, and atomic force microscopy (AFM) demonstrated that the introduction of new oxygen-containing groups or edge-plane defects and the enhanced surface roughness were responsible for the enhanced activity of EGS. Thereafter, a simple electrochemical method for the highly sensitive detection of phenols was established and the detection limits were 0.012 μM, 0.015 μM, 0.01 μM, and 0.04 μM for hydroquinone, catechol, 4-chlorophenol, and 4-nitrophenol, respectively. The facile synthesis of EGS, together with its high electrochemical activity, thus created a novel platform for developing highly sensitive electrochemical sensing systems.