Carbon is a normally used adsorbent for removal of heavy metal ion in aqueous solutions, but the efficient adsorbent needs intensive modification by heteroatom doped or supported noble metals that cause severe pollution and easy leaching of active components during use. In this paper, the role of intrinsic defects on Hg2+ adsorption for carbon adsorbent was investigated. The maximum adsorbing capacity of defect-rich carbon has been improved up to 433 mg·g−1 which is comparable to most of the modified carbon adsorbents via supported metal chloride or noble metal components. The basicity is increased with the content of defective sites and the strong chemical bonding can be formed via electron transformation between the defect sites with adsorbed Hg2+. The present study gives a direction to explore cheap and easily scale-up high-performance mercury adsorbents by simply tuning the intrinsic defective structure of carbon without the necessity to support metal or other organic compounds.