Interconnected porous carbon nanosheets (IPCNs) are promising candidates for use as the electrode materials of electrochemical capacitors, as adsorbents, and as catalytic supports because of their high surface area, efficient ion transport, and three-dimensional (3D) structure. Herein, IPCNs with easily adjustable pore-size distributions, ranging from micro- to mesoporous, are synthesized by the carbonization of alkali metal citrates in a single-step self-templating process. Microporous, mesoporous, and hierarchical micro/mesoporous IPCNs are produced by empolying potassium citrate, sodium citrate, and their mixtures, respectively. Particularly, the hierarchical micro/mesoporous IPCNs derived from the mixture of potassium and sodium citrates in a weight ratio of 8:2 exhibit good electrochemical performance, including high specific capacitance (similar to 200 F/g at 5 mV/s) and excellent rate capability and cyclability. In addition, the structural changes in the carbon materials produced using pyrolysis are thoroughly studied, and the mechanism of IPCN formation is elucidated. The study on the synthetic mechanism of IPCNs provides an important guidance for designing superb porous carbon materials possessing desirable pore structures and particle morphologies for a variety of applications.