Oxidative destruction of organic contaminants of emerging concern (CECs) by photochemical methods has been a topic of high interest in recent years. In order to understand the crucial role of oxidizing radicals, mass spectrometry (MS)-based transformation product studies, either alone or in combination with a suitable chromatographic technique have been widely employed. MS techniques that are developed to explore the photochemical oxidation mechanism of CECs within the past few years have been presented. The necessity of conducting MS techniques along with any advanced oxidation processes (AOPs) targeting to degrade CECs is discussed. The focus is mainly devoted to explaining how the scientific community utilized the results obtained from cutting-edge analytical techniques, such as high-resolution mass spectrometry (HRMS), to explore the identity of major reaction products and thereby, the degradation mechanism of CECs during the event of photochemical AOPs. In order to ensure the environmental safety, it is highly recommended to combine this kind of MS-based mechanistic investigations with any photochemical degradation protocols targeting CECs before their implementation into real environmental uses.