The slow pyrolysis characteristics of lignocellulosic biomass and its three major components via a Thermogravimetric Analyzer coupled with a Fourier Transform Infrared Spectrometer (TGA-FTIR) was studied. Different compositions and ratios of cellulose, hemicellulose, and lignin, olive pomace, sunflower waste, and pinecone were selected. The main decomposition temperature ranges of xylose (hemicellulose) and lignin showed a broad range between 173–690 and 170–835 °C, respectively, whereas that of cellulose was detected to be 291–395 °C. All biomass samples presented a three-stage pyrolysis model that is explained by the superposition of the weight losses of major components. Simultaneous FTIR analysis of the evolved gases demonstrated that the greater the cellulose and hemicellulose contents, the higher the CO and CO₂ concentrations. Chemical kinetics were computed with the Coats–Redfern model. The activation energy required for the initiation of the thermal decomposition of biomass samples is in the range of 53–94 kJ/mol. Moreover, the product yields of all samples were determined via laboratory-scale pyrolysis. Pyrolytic oil and char yields were determined to be between 18.9–32.4 wt.% and 26.6–31.2 wt.%, respectively, at 550 °C final temperature for the biomass samples. It is concluded that the bio-oil yield was not only controlled by the cellulose content but also affected by the presence of n-hexane soluble (oil) fraction as well as inorganics.