The use of hybrid solar thermal devices, which integrate the energy from both concentrated solar radiation and combustion, is receiving growing attention due to their potential to provide a firm and dispatchable thermal energy supply while lowering the costs of energy systems and assisting the penetration of renewable energy. The hybrid solar receiver combustor, HSRC, directly integrates the function of a solar receiver and a combustor into a single device for applications in combined heat and power generation. Compared with the present state-of-the-art of hybrid solar-combustion systems (which collect the thermal energy from the solar and combustion sources in separate devices and then combine them subsequently), the HSRC offers a reduction in total infrastructure (and hence capital costs), levelised cost of electricity, surface area for heat losses and start-up/shut-down losses, as well as lower pollutant emissions, associated with the need to start-up the back-up combustion plant before its heat is required. Its design allows the system to operate in three modes: solar-only (when solar radiation is abundant), combustion-only (in absence of solar energy, with natural gas as the energy source) and a mixed-mode (a combination of both solar and combustion, to manage short and long-term variability of the solar source). This paper presents the first-of-a-kind experimental demonstration of a mixed-mode reaction chamber. A laboratory-scale (20-kWth maximum capacity) HSRC was built and tested under the three different modes of operation. The device was configured to operate with Moderate or Intense Low oxygen Dilution (MILD) combustion to offer low NOx and potential for increased heat transfer. The combustion process was in direct fluid/heat interaction with the environment through the aperture (i.e. no window was employed), when operating under mixed-mode. Natural gas and a 5-kWel xenon-arc solar lamp simulator were used as energy sources for the combustion and solar modes, respectively. The influence of the mode of operation on the thermal efficiency, heat losses and heat flux distribution within the cavity were investigated. Despite the different contributions from the two dominant modes of heat transfer, namely radiation and heat convection, under the different regimes of operation, it was found that the device can achieve similar thermal efficiency in all modes (considering heat recovery from the exhausts). Also, the thermal efficiency was found to increase when operating under mixed-mode in comparison with the combustion-only mode, as a direct effect of hybridisation and due to low heat/mass transfer with the environment (low convective heat losses). The heat flux distributions on the heat transfer fluid coils were found to be significantly different under the different modes of operation.