We identify a new two-dimensional (2D) tetragonal phase of transition metal alloyed boron-carbide (t-CrFeBC) sheet through combined first-principles calculations and Monte Carlo (MC) simulations. The t-CrFeBC sheet prefers a ferromagnetic ground state with the metallic electronic property. Also, the t-CrFeBC sheet is dynamically and thermally stable. t-CrFeBC exhibits sizable magnetic anisotropy energy (MAE) of 120 mu eV per CrFe alloy with an in-plane easy axis (EA) magnetization direction. Moreover, hysteresis loops and other hysteresis related properties (coercivity and remanent magnetization) which are evidence of existence of ferromagnetism in the tCrFeBC sheet are presented for a wide range of temperature. MC simulation results indicate that t-CrFeBC sheet is soft magnetic material with a small coercieve field and narrow rectangular shaped hysteresis curve near the room temperature. All results show that 2D t-CrFeBC sheet holds a unique promise for advanced magnetic device applications.