Managed pressure drilling (MPD) offers a closed-loop circulation system in which formation fracture pressure, bottom hole pressure, and pore pressure are considered and managed at surface. The right choice of drilling fluid used during MPD operation facilitates proper mud management and treatment. Lab formulation and field trial of a high-density water-based drilling fluid comprising a mixture of barite (BaSO4) and manganese tetroxide (Mn3O4) as weighting agents have been described in this paper. Drilling fluids having a mixture of Mn3O4 and BaSO4 as weighting agents would have lower equivalent circulating density (ECD), better sag, better acid solubility, and lower fluid cost as compared to conventional BaSO4-based fluids. This paper describes the formulation of three different water-based drilling fluids viz. 100, 120, and 150 pcf drilling fluids having a mixture of Mn3O4 and BaSO4 and hot rolled at temperatures of 270, 250, and 300 °F, respectively. Rheological properties, sag resistance, and high pressure-high temperature (HPHT) filtration properties of the three fluids have been described in the paper. Data obtained from the field trial of 160 pcf high-density drilling fluids having a mixture of Mn3O4 and BaSO4 for wells with a 300 °F bottom hole static temperature have been described. HPHT operations across naturally fractured formations with 0.5–1.0 pcf drilling fluid window have been described in the paper. During the field trial, the fluid having a mixture of Mn3O4 and BaSO4 showed good rheological, filtration, and sag properties thereby resulting in successful drilling of the well with no issues. MPD operation became more successful and practical with high-density drilling fluids having a mixture of Mn3O4 and BaSO4 as it facilitated better drilling fluid management and treatment in comparison to conventional fluids.