The structural, mechanical, and electronic properties of C2/m-carbon were studied utilizing the first-principles calculations. The calculated lattice parameters and elastic constants of diamond are in excellent agreement with the available experimental data, indicating our calculations for C2/m-carbon are valid and believable. The calculated elastic constants indicate that C2/m-carbon is mechanically stable according to the elastic stability criteria under pressure. Furthermore, the elastic anisotropy has been visualized in detail by plotting the directional dependence of Poisson's ratio, Young's modulus, and shear modulus, whereas the calculated values of Poisson's ratio and B/G present their brittle manner. B/G increases under increasing pressure with B/G = 1.75 at about 260.74 GPa and v increases observed with increasing pressure with v = 0.26 at about 261.35 GPa for C2/m-carbon, respectively. Our calculations predict that C2/m-carbon is an indirect semiconductor with wide band gap of 4.197 eV.