The thermal resistance of a high electron mobility transistor (HEMT) forming part of a monolithic microwave integrated circuit (MMIC) is noninvasively extracted under real working conditions (electrical and thermal) by infrared thermal imaging. The HEMT thermal resistance considers the device local maximum temperature and dissipated power. An experimental approach to this end is currently not available, as the HEMTs thermal interaction does not allow extracting its individual heat generation. Thanks to thermal field confinement offered by heat source frequency modulation, the power dissipation in each device is inferred, making feasible its individual thermal resistance extraction. As a result, reasonable values of the local thermal resistance of each individual HEMT integrated in the MMIC (i.e., 57.8 ± 3.4 °C/W and 24.8 ± 1.4 °C/W) are obtained in agreement with studies on discrete devices available in the literature.