The number of 2D systems based on Mn and C atoms has been growing significantly due to the magnetic and electronic properties that these monolayers present. In this work, a new Mn-C monolayer by density functional theory is proposed. The MnC4 stability is studied by the formation energy formalism to compare with other reported Mn-C monolayers. Our analysis points that the MnC4 monolayer stabilizes at extremely rich C conditions. Also, the MnC4 monolayer possess positive phonon branches, evidencing its dynamic stability. Further, it confirms that this monolayer is also stable up to 300 K. This new monolayer has an antiferromagnetic alignment with spin-channels along the [-110] direction and metallic behavior, being a strong candidate in the formation of exchange-biased systems when forming a van der Waals heterojunction with other Ferromagnetic materials.