We investigated the physicochemical properties of biochar produced at 500, 600, and 700 °C from sewage sludge (SSL) and SSL with willow (8:2 and 6:4, w/w). The effect of carrier gas type (N2 or CO2) on the biochars properties was examined. The pH, electrical conductivity (EC), specific surface area (SBET), elemental composition (CHNO), and mineral composition (XRD), as well as functional groups (FT-IR/PAS) were identified and determined. Replace the N2 by CO2 as carrier gas influenced the properties of the all produced biochars. CO2 had an effect on reducing the process efficiency as well as pH and EC in the SSL-derived biochars. Changes in the elemental composition (H and O content) were also noted after change of carrier gases. In the biochars obtained in a CO2, a negative correlation was found between EC and pyrolysis temperature. In the biochars produced from SSL and willow, apart from similar changes to those observed for the biochars made from SSL in the presence of CO2, changes in the surface properties were additionally noted. The biochar obtained in a CO2 atmosphere had higher SBET values (from 19 to 47%) than the respective biochars produced in an N2 atmosphere.