Livestock waste is an ideal precursor for preparing carbon-based catalysts and its conversion into functional biochars for wastewater treatment may offer dual benefits for waste reutilization and environmental remediation. Herein, boron and nitrogen co-doped porous biochars (BNPBs) were successfully synthesized using a mixture of yak dung, boric acid, and urea via a one-step pyrolysis. The co-doping of B and N atoms into the biochars regulated their porosity, defect degree, and surface functional groups, and thereby facilitated the activation of peroxymonosulfate (PMS) for efficiently treating 3-aminophenol (3-AP). The catalyst BNPB-700 exhibited the optimal catalytic performance, and achieved 85.3% removal efficiency of 3-AP within 30 min. Mechanism analyses suggested that reactive oxygen species (ROS) including SO₄•⁻, •OH, O₂•⁻, and ¹O₂ together with direct electron transfer cooperatively contributed to the 3-AP removal through both mineralization and polymerization pathways. Application to the treatment of real sheep and pig manure wastewaters further confirmed the practical applicability of the BNPB-700/PMS system. By coupling the upcycling of yak dung into B, N co-doped biochars and the utilization for wastewater treatment, this work provides a promising strategy for achieving “waste to treat waste”.