Plant polyphenols are reported to have bioactive properties, which may be used for protection against diseases. Therefore, the aim of this research was to investigate the antioxidant activities of a pomegranate tannin polyphenol compound, punicalagin and pomegranate juice. The presence of punicalagin in pomegranate husk (US) and pomegranate juice (US & UK) was compared with a punicalagin standard using high performance liquid chromatography (HPLC) and liquid chromatography-mass spectroscopy (LC-MS) which are highly sensitive and selective analytical methods for the separation and identification of phenolic compounds and anthocyanins. Antioxidant mechanisms involving DPPH radical scavenging activity, hydrogen peroxide scavenging, ferrous chelating and reducing ability were also studied on pomegranate juice and standard punicalagin. The present study shows a high degree of similarity of HPLC and LC-MS results between the punicalagin commercial standard (Sigma Aldrich) and US pomegranate husk extracted with methanol. In contrast, in the methanol juice extract obtained from US and UK, higher hydrogen peroxide scavenging activity was achieved by 0.1 mg/ml from both punicalagin and pomegranate juice when compared with butylated hydroxytoluene (BHT) or trolox (p ≤ 0.01). Punicalagin and pomegranate juice exhibited ferrous chelating ability significantly lower than Ethylenediaminetetraacetic acid. These findings confirmed that punicalagin was present in pomegranate husk compared to pomegranate juice, as measured using a punicalagin standard. The antioxidant mechanism experiments concluded that, the pomegranate juice has a significantly higher radical scavenging activity in comparison with punicalagin (p ≤ 0.01). However, punicalagin showed significant ferrous chelating activity and reducing power ability in a dose-dependent manner as compared with pomegranate juice.