Background: Lentil (Lens culinaris M.) is a legume widely consumed worldwide. It is rich in bioactive compounds, including polyphenolic compounds that contribute to positive health benefits. Methods: This study aimed to determine the phenolic content and antioxidant activity of black, red, green, and brown whole lentils. Towards this end, the lentils’ phenolic compounds were evaluated regarding their total phenolic content (TPC), total flavonoid content (TFC), total tannin content (TTC), total condensed tannin (TCT), total proanthocyanin content (TPAC), total anthocyanin content (TAC). For the antioxidant activity 2,2-diphenyl-1-picrylhydrazyl (DPPH), ferric reducing antioxidant power (FRAP), 2,2′-azinobis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS), hydroxyl radical scavenging activity (•OH-RSA), ferrous ion chelating activity (FICA), reducing power assay (RPA) and phosphomolybdate (PMA) assay were accessed. To identify individual phenolic compounds, liquid chromatography-electrospray ionization quadrupole time-of-flight mass spectrometry (LC-ESI-QTOF-MS{}^2) was used. Results: The results showed that green lentils exhibited the highest TPC (0.96 mg gallic acid equivalents (GAE)/g) whereas red lentils presented the highest TFC (0.06 mg quercetin equivalents (QE)/g). Black lentils were noted with the highest TCT (0.03 mg catechin equivalents (CE)/g), TPAC (0.009 mg cyanidin chloride equivalents (CCE)/g), and TAC (3.32 mg/100 g) contents. While the greatest TTC (2.05 mg tannic acid equivalents (TAE)/g) was observed in the brown lentil. Regarding the total antioxidant capacity, red lentils (4.01 mg ascorbic acid equivalents (AAE)/g) presented the greatest activity, whereas the lowest was found in the brown samples (2.31 mg AAE/g). The LC-ESI-QTOF-MS{}^2 tentatively identified a total of 22 phenolic compounds, containing 6 phenolic acids, 13 flavonoids, 2 lignans, and 1 other polyphenol. The relationships among phenolic compounds by Venn Diagram showed a high number of overlapping compounds in brown and red lentils (6.7%), and a low number of overlapping compounds between the green, brown, and black lentils (2.6%). Flavonoids were the most abundant phenolic compound within the studied whole lentils, with the brown lentils being the richest in phenolic compounds, especially flavonoids. Conclusions: This study emphasized a comprehensive understanding of the antioxidant potential of lentils and disclosed the phenolic distribution across various lentil samples. This may increase interest in the development of functional food products, nutraceutical ingredients, and pharmaceutical applications with lentils.