The ground state of a molecular diamond lattice compound (ET)Ag-4(CN)(5) is investigated by magnetization and nuclear magnetic resonance spectroscopy. We found that the material exhibits a Mott-insulating ground state with antiferromagnetic long-range ordering at 102 K. The ordered moment shows weak ferromagnetism with a tiny canting angle. The spin susceptibility is well fitted into the diamond lattice Heisenberg model with a nearest-neighbor exchange coupling of 230 K, indicating the less frustrated interactions. The transition temperature elevates up to similar to 195 K by applying pressure of 2 GPa, which records the highest temperature among organic molecular magnets. The first-principles band calculation without electron correlations suggests that the system is accessible to a three-dimensional topological semimetal with nodal Dirac lines, which has been anticipated on a half-filling diamond lattice.