© 2021 Elsevier LtdCareful designing of biochar (BC) employing biological and industrial waste materials has gained much attention to improve soil health via reducing the bioavailability of heavy metals (HMs) in the contaminated-soil. Herein, a forty-day pot experiment was conducted to explore the influence of micro-plastic (MP) embedded sewage-sludge (SS). The effects of SS and MP dosage ratios (0–15%) on the physicochemical properties of the modified-biochars were also studied with slow-pyrolyzed (SBC-500 °C) and flash-pyrolyzed biochar (FSBC-700 °C). Our investigations found remarkable positive synergistic effects for the SBC-15% involving increased soil pH, CEC, high carbon contents, and alleviation in Cr and Pb leaching than control and FSBC-700 °C. Besides, SBC-15% containing high carbon functional groups can effectively mitigate the Cr and Pb availability stress by intensifying the adsorption or passivation in the amended-soil, thereby, significantly reducing the Cr and Pb EDTA-extractable contents. Chemical fractionation analyses further confirmed that SBC-15% addition was more helpful for Cr and Pb immobilization and ultimately reducing transfer-rate, bioconcentration-factor, and translocation-factor as compared to FSBC-15% and control due to its higher alkalinity, surface area/porosity, and available carbon functional groups. The maize biomass (root and shoot) increased by more than 50%, and the activities of soil enzymes such as urease, alkaline phosphatase, and glucosidase enzyme activities were also enhanced. This ecologically feasible strategy would pave an efficient way to make full use of the SS and MP for the biochar synthesis with excellent soil remediation performance.