This paper assesses the stages and the field application results of a slope monitoring system developed for continuous long-term monitoring applications using an optical fiber system to decrease the risks associated with slope instabilities. During this study, a system composed of optical fiber cables and a device referred to as the Brillouin Optical Time Domain Analyzer (BOTDA) have been used. The system is a distributed system that measures the strain along an optical fiber cable when slope instability displaces the cable and causes differentiation of the initial location of the cable. This system has been implemented on a risky unstable slope in the Bahçecik region of Kocaeli, one of the active landslide-prone regions in the Marmara region, Türkiye. The strain data collected through this real case field application has enabled the investigation of the effects of hydrological and hydrogeological conditions along with seismic activity on slope movement for a period of 12 months. For this purpose, in addition to the optical fiber system, a piezometer and an acceleration seismometer were deployed in the field. The evaluation of the data revealed that the change in strain was directly proportional to precipitation and groundwater head. Moreover, the seismic activity also influenced the deformation as a secondary triggering effect. In addition, the slope movement characteristics were validated by surface topography monitoring measurements. The results showed that utilization of the optical fibers is reasonable and the system is well-suited for developing monitoring systems for deformation-related engineering applications such as slope instabilities.