Receiver level Global Navigation Satellite Systems (GNSS) Signal and Measurement Quality Monitoring (SQM and MQM) to detect and de-weight measurements distorted by multipath are investigated. SQM and MQM monitoring metrics are defined at the tracking and measurement levels of the receiver; a new geometry-based de-weighting technique is developed. Following an analytical discussion of the sensitivity and effectiveness of the metrics, field data analysis is provided for static and kinematic modes to verify practical performance. Results obtained for the designed SQM and MQM-based detection metrics show reliable performance of 3 to 5 m Minimum Detectable Multipath Error (MDME). Although limited by multipath characteristics and measurement geometry, when detected faulty measurements are de-weighted, positioning performance improves by up to 53% for different multipath scenarios.