Nature remains a valuable source for many phytochemicals that provide different physiological activities. Citrus reticulata (Rutaceae) or Mandarin is a widespread plant used worldwide in food, beverages, perfumes, and folk medicine, including cancer prevention, flu treatment, inflammation, and generally in body healing and strengthening. Citrus reticulata have also been used for treating indigestion, bronchitis and asthma. Phytochemical investigation of the Citrus genus has revealed the existence of a broad range of bioactive compounds, including flavonoids, flavonoid glycosides, sterols, fatty acids, and alkaloids, along with many other miscellaneous compounds. Among these diverse bioactive compounds, flavonoids (e.g., polymethoxylated flavones, glycosylated flavanones, flavonoids and limonoids), were found to be the largest group of phytochemicals in the Citrus genus. These flavonoids have been reported to have antioxidant, anti-inflammatory, anti-allergic, anti-atherogenic, anti-proliferative, antibacterial, antiviral, and antifungal activities. Additionally, they have a role in preventing atherosclerosis as they reduce total blood cholesterol, showing also anti-hypertensive, hypolipidaemic, diuretic and analgesic properties. This review study summarizes the secondary metabolites identified in Citrus reticulata (Rutaceae) or Mandarin.