The use of wood ash as an additive in biochar production was shown to increase biochar yields and was suggested to improve the recycling of ash-derived nutrients. However, there is limited knowledge on the interaction of ash addition with pyrolysis conditions and their effects on biochar yield and properties. Here, we performed experimental pyrolysis in an auger reactor applying different ash concentrations, temperatures, and residence times. Biochar yield (dry and ash-free) increased by 26% and carbon-conversion efficiency of pyrolysis by 36% when 9 wt % ash was added to softwood. Up to this concentration, yield increase was correlated linearly, while higher ash concentrations did not further increase yield significantly. The ash-induced yield increase was lower at 400 °C than for 425−500 °C. Biochar’s content of polycyclic aromatic hydrocarbons was not affected by ash addition. The content of polychlorinated organic pollutants increased with ash amendment but remained well below applicable thresholds. The electron exchange capacity of biochars was increased while the micropore specific surface area and thermal stability decreased with ash addition. The elevated potassium content of ash-amended biochars promoted sunflower growth. Biochar derived from ash-amended biomass is safe to be applied in soil and may be a promising approach for enhanced nutrient recycling and carbon sequestration.