Due to the extremely low C/N ratio, high concentration of ammonia nitrogen and refractory organic matter of mature landfill leachate (MLL), appropriate processes should be selected to effectively remove nitrogen and reduce disposal costs. Partial nitritation (PN) and anaerobic ammonia oxidation (AMX) have been used as the main nitrogen removal processes for MLL, and the sludge granulation in PN and AMX processes could contribute to high biological activity, good sedimentation performance, and stable resistance to toxicity. In this study, the O-PN-AMX biogranules process was selected to effectively remove nitrogen from MLL without carbon addition and pH adjustment. Without uneconomical NH-N oxidation and wasting the alkalinity of MLL, ozone pretreatment achieved color removal, decreased humic- and fulvic-like acid substances, and alleviated the MLL toxicity on ammonia oxidizers. In addition, the ozonation of MLL could shorten the start-up time and improve the treatment efficiency and biogranules stability of PN and AMX processes. Efficient and stable nitritation was achieved in PN reactor without strict dissolved oxygen (DO) control, which was attributed to the unique structure of granular sludge, ozone pretreatment, and alternating inhibition of free ammonia and free nitric acid on nitrite oxidizers. Through the application of ozone pretreatment and granular sludge, the nitrogen removal rate (NRR) and nitrogen removal efficiency (NRE) of the O-PN-AMX biogranules process reached 0.39 kg/m/day and 85%, respectively, for the undiluted MLL treatment. This study might provide a novel and effective operation strategy of combined process for the efficient, economical, and stable nitrogen removal from MLL.