Three species of macroalgae were treated with the aim of reducing nitrogen, sulfur and ash within the biomass prior to hydrothermal processing. The treatments were the nutrient starvation of cultures and post-harvest washing of biomass in freshwater. Subsequently, hydrothermal liquefaction (HTL) of macroalgae was carried out in a batch reactor heated for 8 min with a maximum temperature of 345 °C. Nutrient starvation effectively reduced nitrogen and sulfur levels within the biomass, which led to a reduction in nitrogen by 51–59 wt.% and sulfur by 64–88 wt.% within the biocrude. The yield of biocrude was highest for Derbesia at 38.6–41.7 wt.% and Oedogonium at 35.6–38.8 wt.% when not starved, but was reduced by up to 19 wt.% when the biomass was starved. The washing of biomass consistently reduced the ash content for all species by 7–83 wt.%. The removal of ash affected neither the quality nor the quantity of biocrude produced. The two treatments demonstrate that macroalgal biomass can be effectively manipulated in the production process to modify the composition of the feedstock and, consequently, improve the quality of biocrude. Additionally, reducing the ash content of biomass minimizes its potential impact on HTL processing equipment.