In this paper, the potential of flax shives use as sustainable materials for environmental and energy applications is assessed. In particular, raw flax shives and their pyrolytic chars produced at three different temperatures (400, 500, and 600 °C) are fully characterized using several analytical techniques including thermogravimetric analysis (TGA), X-ray fluorescence (XRF), scanning electronic microscopy (SEM), CO ₂ adsorption, Raman spectroscopy, and DRIFT (diffuse reflectance infrared Fourier transform) spectroscopy. Furthermore, thermogravimetric analyses and their derivative (TGA/DTG) during the thermal degradation of the flax shives were used to deduce the corresponding kinetic data. These latter were calculated using three different models: Friedman, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS). The main results indicate that the raw flax shives have high potential in co-firing, which is very close to usual energy vectors in some rural contexts and even more enhanced compared to many other biomasses used in industrial fields. They can also be an attractive carbon source for agricultural soils. Furthermore, the chars derived from flax shives contain important rates of mineral species such as calcium, potassium, magnesium, and phosphorous, which allow their reuse as low-cost amendments for agricultural soils. At the same time, the surface area and the microporous structure of the flax shives biochars are developed enough allowing them to be used as effective adsorbents for pollutants contained in gaseous phase. After an activation step, these biochars’ physico-chemical properties could be significantly improved permitting their application as promising materials for soil bioremediation or wastewater treatment.