A compound two-dimensional (2D) monolayer mixing Mn atoms and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules was synthesized by supramolecular assembly on a Cu(100) surface under ultrahigh vacuum conditions. Its structural, electronic, and magnetic properties were analyzed by scanning tunneling microscopy experiment and theory, low-energy electron diffraction, x-ray photoemission spectroscopy, and density-functional theory calculations. The 2D compound has a long-range ordered square planar network structure consisting of fourfold coordinated Mn centers with a Mn: TCNQ ratio of 1:2. The electronic-state analysis revealed a complex charge-transfer scenario indicating strong bonding of TCNQ with both the Mn adsorbates and the Cu surface atoms. The calculations reveal that the Mn centers carry a magnetic moment close to 5 mu(B) with very weak coupling between adjacent Mn centers.