The photothermal effect of metal nanoparticles embedded in polymeric materials can be used to efficiently generate local heat for in situ thermally processing within an existing material. Fluorescent probes are employed as thermal sensors to allow dynamical measurement of the amplitude and rate of temperature change within the polymer matrix. The efficacy of this technique is demonstrated in polymer nanocomposite samples with different morphological characteristics, namely nanofibrous mats and thin film samples. For similarly thick materials and both types of sample morphology, average temperature increases on the order of ≈100s °C are readily obtained with dilute nanoparticle concentrations under relatively low irradiation intensity. Thus, the in situ photothermal heating approach has great potential for controllably driving a multitude of thermal processes, such as triggering phase transitions, generating site-specific cross-linking, or initiating chemical reactions from within a material.