In this paper, we describe the design, modeling, and experimental testing of a film cooling scheme employed on an unshrouded high-pressure (HP) rotor casing. The casing region has high thermal loads at both low and high frequency, with the flow being dominated by the potential field of the rotor and over-tip leakage flows. Increasingly high turbine entry temperatures necessitate internal and film cooling of the casing to ensure satisfactory service life and performance. There are, however, very few published studies presenting computational fluid dynamics (CFD) and experimental data for cooled rotor casings. Experimental testing was performed on a film-cooled rotor casing in the Oxford Turbine Research Facility (OTRF)—a rotating transonic facility of engine scale. Unsteady CFD of an HP rotor blade row with a film-cooled casing was undertaken, uniquely with a domain utilizing a sliding interface in the tip gap. A high density array of thin film heat flux gauges (TFHFGs) was used to obtain time-resolved and time-mean results of adiabatic wall temperature and film cooling effectiveness on the film-cooled rotor casing between −30% and +125% rotor tip axial chord. Results are compared to CFD predictions, and mechanisms for interaction of the coolant with the rotor tip are proposed and discussed. Acoustic effects within casing coolant holes due to the passing of the rotor are demonstrated on a 3D CFD geometry, supporting conclusions drawn in earlier work by the authors on the importance of this effect in a casing film cooling system.