Waste management to reduce the loss of natural resources has become a basis of sustainable development and a circular economy. When using waste, the heavy metal (HM) concentration must be taken into account since HMs can be potentially released to the environment, posing a toxicity threat. The aim of the study was thus to estimate the availability for plants of Cr, Ni, Cu, Zn, Cd, and Pb introduced into the soil with waste. We hypothesized that the prepared waste mixtures containing coal or biomass ash and municipal sewage sludge would reduce the environmental risk compared to the studied waste used separately. The research was conducted during a 6-year field experiment with grasses and legumes. HM concentration in soil, waste, and plant biomass; tolerance index; and uptake of HMs by plants were measured. The ash–sludge mixtures had a more favourable effect on the soil in terms of pHKCl, TOC, total nitrogen, and total exchangeable bases than the waste used separately. This provided beneficial conditions for plant growth and development. Consequently, the ash–sludge mixtures increased the plant yield as compared to ash alone, while the mixture containing the biomass ash also enhanced the yield in relation to the sewage sludge. The study showed that the mixtures allowed for a reduction of environmental risk arising from the HM input with waste to the soil. It was proven that HM availability for plants could be beneficially modified by mixing waste. Combining the coal ash with the sewage sludge is particularly recommended, owing to the unfavourable properties of coal ash for plants. The application of the higher dose of the coal ash–sludge mixture showed a better effect than the lower dose, while the influence of both doses of the biomass ash–sludge mixture was similar. Under the ash–sludge treatment, plants took up more HM than under the ash used separately, and the HM concentration in the obtained biomass did not generally exceed that observed under single wastes. This should reduce the accumulation of HMs in the soil during a long-term use of the waste and facilitates the utilisation of the produced biomass