High quality, micron-sized interpenetrating grains of MgB₂, with high density, are produced at low temperatures (~420 °C < T < ~500 °C) under autogenous pressure by pre-mixing Mg powder and NaBH₄ and heating in an Inconel 601 alloy reactor for 5-15 h. Optimum production of MgB₂, with yields greater than 75%, occurs for autogenous pressure in the range 1.0 MPa to 2.0 MPa, with the reactor at ~500 °C. Autogenous pressure is induced by the decomposition of NaBH₄ in the presence of Mg and/or other Mg-based compounds. The morphology, transition temperature and magnetic properties of MgB₂ are dependent on the heating regime. Significant improvement in physical properties accrues when the reactor temperature is held at 250 °C for >20 min prior to a hold at 500 °C.