Superconducting (SC) MgB₂ thin films were prepared in situ in one step by vacuum co-deposition at extremely low growth temperature (T_s  ∼  110-150 ^°C) and at very low Mg deposition rate, far off from the MgB₂ stability regime of the thermodynamic phase diagram. As-grown films are nanocrystalline and exhibit (002) texture, moderate surface roughness and SC onset up to T_c ^on = 18 K. This is the highest critical temperature ever observed in low-T_s as-grown MgB₂ films. Our preparation method at very low T_s is favorable for the fabrication of layered SC hybrid electronic devices, where deposition and post-annealing at high temperature are detrimental to device functioning. Simple post-annealing of our films in ultrahigh vacuum enhances T_c ^on up to 22 K due to the improvement in structural properties.