Global Navigation Satellite System (GNSS) Radio Occultation (RO) is an atmospheric remote sensing technique that improves global weather forecasting, climate monitoring, and ionospheric studies. Planetary boundary layer height (PBLH) is a crucial parameter in modeling the troposphere. Space-based GNSS RO has been used in detecting the PBLH with receivers onboard low earth orbit satellites. This paper presents a method of PBLH detection using GNSS signal amplitude measured by a mountaintop-based RO (MRO) system on the summit of Haleakala, Hawaii. The estimated PBLHs are comparable with those derived from space-based RO measurements, space-borne lidar, and local radiosonde profiles. With advantages such as having dense temporal and spatial coverage, low-cost, and an easy-to-implement algorithm, the MRO-based signal amplitude method can be a useful addition to existing methods and could contribute to regional weather study.