The aim of this study was to evaluate the performance of horizontal sub-drains (HDs) installed to prevent landslides. The process was based on a design of HDs on slopes to control rainwater infiltration to ensure stability. This was in the form of 72 HDs, between 5–15 m long, installed at the end of 2021 to measure water discharge during the early rainy seasons of 2022 and 2024. Moreover, geophysical tests were conducted through ERT and the water table was monitored during wet and dry seasons apart from other fields and laboratory geotechnical investigation. The results showed that only 28 of the 72 installed HDs were discharging water in early 2022 and the number decreased to 17 in 2024 from a flow rate of 0.08 L/min to 7.5 L/min respectively. Geophysical tests (ERT) showed water accumulation zones at specific depths due to rainwater infiltration and HDs effectively discharged accumulated water. Meanwhile, HDs installed in areas lacking water accumulation zones with limited lengths were ineffective in water removal. These field results correlated with numerical modeling conducted using coupled Limit Equilibrium Method (LEM) and Finite Element Method (FEM) programs regarding the impact of rainwater seepage on soil and the efficacy of HDs. The trend showed the importance of appropriate HDs placement and length, particularly in regions prone to water accumulation. Furthermore, the ERT test conducted during different seasons proved useful in identifying water accumulation zones and determining the optimal location and length of HDs to be installed to mitigate landslides triggered by rainfall.