The effect of three operating parameters (peak temperature, pressure, and pyrolysis atmosphere) during the slow pyrolysis of three biomass sources (corn stover, vine shoots, and two-phase olive mill waste) was extensively analyzed. A 2-level full factorial design of experiments was adopted to assess the effect of the above-mentioned factors on the potential stability of biochar as well as the yields of the main pyrolysis products. To evaluate the effect of the biomass feedstock, the design was divided into three blocks (one per biomass feedstock). Results from the statistical analyses indicated that the properties of biochar related to its potential stability were mainly affected by the peak temperature and, to a lesser extent, the biomass feedstock. A significant increase in the yield of produced gas was observed when pressure was raised. This increase in the total gas yield was in part due to a higher release of CO, CH4, and H2. Using a pyrolysis atmosphere of CO2 (instead of N2) did not result in any remarkable change in neither the distribution of the pyrolysis products nor the potential stability of biochar. However, when CO2 was used as carrier gas, a significant increase in the yield of CO, at the expense of produced CO2, was observed. The findings reported herein suggest that processing biomass through pressurized slow pyrolysis under CO2 atmosphere is interesting to simultaneously obtain two valuable products: a biochar with an appropriate carbon sequestration potential, and a produced gas with an appropriate composition for energy recovery purposes.