The Tuaheni Landslide Complex (TLC) on the Hikurangi Margin is subject to reactivation, yet the timing of slide emplacement remains unknown. Here we modeled pore-water data collected from the TLC during the International Ocean Discovery Program (IODP) Expedition 372 in 2017/2018 using a transient-state reaction-transport modeling approach. Our simulations reveal that the TLC was formed by two separate depositional events and that the most recent one occurred 12.5 ± 2.5 ka as a coherent ~40 m sediment block that carried its initial pore-water signature. In addition, we show that the rapid burial of pore-water SO42− in the pre-slide sediments consumed an additional ~5.6 × 109 mole of CH4 over the entire Tuaheni South following the most recent depositional event. These findings provide significant insights into the nature and timing of the TLC and highlight the role of slope failure on subsurface methane cycling on millennial timescales.