Hydrothermal liquefaction (HTL) is a promising technology with huge potential for the chemical recycling of plastic waste. Herein, a two-stage sequential HTL process is proposed for the conversion of acrylonitrile–butadiene–styrene (ABS) plastic to produce energy-dense oils with minimal nitrogen content. In the first step, ABS is denitrogenated using subcritical water (250–350 °C) and an alkaline potassium hydroxide (KOH) catalyst to produce solid product 1 (SP1) and aqueous phase 1 (AP1). A 95% denitrogenation efficiency of ABS is achieved at 325 °C in 20 min, with a catalyst load of 0.30 grams of KOH per gram of ABS. Under these conditions, the nitrile bonds in ABS are hydrolyzed to form a potassium– ABS salt complex that migrates to AP1. This AP1 is then neutralized with hydrochloric acid (HCl) to produce the denitrogenated solid product 2 (SP2). The nitrogen from ABS is easily removed as ammonia (NH3) by evaporation. SP1 and SP2 from stage 1 are then liquefied at the second hydrothermal step (375–425 °C, 250 bar) to produce low-nitrogen-containing oils with high heating values up to 41.6 MJ∙kg−1. The oils primarily consist of mono-, di-, and poly-aromatic compounds, with boiling ranges similar to those of kerosene and vacuum gas oil. The gases from the supercritical HTL contain C2–C5 hydrocarbons. The results indicate that the two-stage sequential HTL is successful in removing nitrogen from the ABS and producing nitrogen-free aromatics, with much greater efficiency than the single-stage supercritical HTL of ABS.