Phenolic compounds during recent decades have been the object of study by the scientific community due to their high and diverse bioactive potential, with their antioxidant capacity being one of their most studied properties since, due to their structure, they have a high potential to act as effective compounds against oxidative stress. Due to their great structural diversity, these compounds have been classified into at least 10 different classes based on their basic chemical structures. The aim of this work was to carry out the characterization of the main phenolic content and the determination of the antioxidant capacity of four plant matrices. Firstly, Lippia citriodora, Hibiscus sabdariffa, Olea europaea, and Silybum marianum were chosen as natural sources due to their high content of phenolic compounds. After that, a qualitative characterization of the phenolic profiles of the selected plant extracts was performed using high-performance liquid chromatography coupled with mass spectrometry. Finally, the determination of the total content of polyphenols was carried out using the Folin–Ciocalteu method, and the antioxidant capacity using electron transfer methods (FRAP and TEAC) and hydrogen donation (ORAC). After the chromatographic analysis of the phenolic profile of each matrix, it was observed that there are different major phenolic compound families for each plant matrix. Regarding the antioxidant capacity, L. citriodora showed better results for the FRAP assay, while for the TEAC and ORAC assays the highest values were for S. marianum.