Natural matters have provided clues to advanced and pragmatic material designs. Implementing their molecular structures or functional moieties to artificial materials can provide blueprints to unprecedented functionalities. Here, we report a flexible biomimetic thermal sensing (BTS) polymer that is designed to emulate the ion transport dynamics of a plant cell wall component, pectin. Utilizing a simple yet versatile synthetic procedure, elastic fragments are inserted into a block copolymer architecture, engineering its mechanical properties such as flexibility and stretchability. The thermal response of our flexible BTS polymer outperforms current state-of-art temperature sensing materials, including vanadium oxide, by up to two orders of magnitude. Thermal sensors fabricated from the composite material exhibit a sensitivity that exceeds 10 mK and operate stably between 15 to 55 ℃, even under repeated mechanical deformations. We exemplify the use of the flexible BTS polymer composite in arrays for spatio-temporal temperature mapping and broadband, long-wave infrared photodetection.